Droids have replaced stormtroopers in The Phantom Menace as the go-to minions for two major reasons—neither of which have to do with either's ability to hit their target.
The first reason is entirely practical. To secure a family-friendly PG rating, Lucas couldn't have the Jedi's lightsabers slicing and dicing through minions that are actual people. That'd be a grindhouse level of over the top violence that would make even Quentin Tarantino blush.
But we don't mind watching the Jedi julienne robots… because they aren't human.
This segues nicely into the second reason, which is that the droid army symbolizes the inhuman methods of the antagonists. The term "inhuman" means to lack the qualities of a human being, specifically sympathy, pity, warmth, and compassion. The droid army represents this lack of qualities perfectly because they ain't human.
They can't feel pity or compassion or sympathy. They aren't designed to do so, and they can't learn what is outside their programming. Instead, the battle droids are programmed to perform only two functions: fight and obey the orders without question. They can't be reasoned with or be made to feel sympathy anymore than a gun because they're tools. (Both in the literal and figurative sense.)
The perfect image to summarize this comes right before the droids engage the Gungans on the battlefield. Realizing their barrage can't get through the Gungan shields, the troop transports send foot soldier droids to fight. The droids are removed from the transport on a rack that looks like something from a factory line.
The droids are placed on the ground one at a time and then activated. It's a cold, mechanical birth, and as soon as they "come to life," they have one purpose and one purpose only. They aim to kill them some Gungans… which they do poorly.
Hey, we said they were tools; we didn't say they were effective tools.
Like Builder, Like Robot
Like the droid army, the methods and objectives of the antagonist are inhuman. Neither Darth Sidious nor Viceroy Gunray considers how their actions will harm the Naboo or others. They invade an entire planet and starve the population because they take issue with… space taxes.
Yeah, that's right. They don't like paying the intergalactic version of the IRS. And both are willing to kill, maim, injure, and perform all manner of horrible acts to get what they want.
A perfect example of this is Gunray's exchange with Governor Bibble:
"Your queen is lost, your people are starving, and you, Governor, are going to die much sooner than your people, I'm afraid."
We never see a scene featuring Gunray philosophizing that perhaps killing a man and starving his people is wrong. Nope. For him this is just how you do business. Like his droid army, the character is completely lacking in sympathy for others.
Compare this to our heroes. Qui-Gon tried to free Anakin and Shmi because he felt compassion for them and their plight. While he was only able to free Anakin, his motives came from a place of humanity. He could have easily stolen the hyperdrive parts he needed from Watto, probably killing the junk dealer in the process, but that would have been cruel and inhumane. Instead, he let Watto's shortcomings unravel him, even teaching the Toydarian a lesson in the dangers of gambling. (Don't gamble, kids.)
So when these two forces meet in battle, it is a symbol of the film's philosophical conflict as well. The droids aren't human and so represent our more pitiless and cold-blooded motivations. On the other hand, the heroes are all human—or, at least, they're sentient creatures able to feel emotions—and so symbolize the better qualities of human motivations, like empathy, compassion, and teamwork.
Nature vs. Circuit Board
The droid army also represents a theme that underlies most of the Star Wars films—the superiority of nature over technology.
It's a bit ironic given all the technology used to make these films, but each Star Wars movie has really driven home the idea that living in harmony with nature is preferable to being technologically separated from our environment.
The droids represent technology in this equation. Being unnatural, they don't care much for nature or animals—just check out the way they crash through the forest when they first arrive on Naboo.
On the other hand, our heroes live closely with nature. The Gungans live in an underwater city that blends into the natural environment. The Naboo build their cities and palaces to highlight the natural beauty surrounding them. And the Jedi try to live in harmony with all living things through the Force.
When the Gungans and droids battle, it's a fight of nature vs. technology. Even the Gungan weapons are constructed with natural elements, including pack animals and electric globs of natural… goo. Ultimately, nature triumphs when the droids are defeated.
Now if we could just do something about making those electrical globs of goo less icky…