As with all the characters in Gulliver's Travels except for the title character, Mr. James Bates exists mainly as a name, without any particularly distinctive traits. He helps Gulliver set up his medical practice in London, which goes south once Mr. James Bates dies. The one thing really illuminating about this guy is that he shows us something about Swift's sense of humor. Gulliver won't stop referring to the guy as his "Master Mr. Bates," or, more directly, "my good master Bates" (1.1.3). Read that last one aloud, and you'll see the joke. Swift makes a ton of fairly broad jokes about human bodies in this book, from the giant servant girl who likes to balance Gulliver on her nipple to Gulliver's putting out a Lilliputian fire by peeing all over it. This book ain't prudish, is what we're saying, and Gulliver's good master Bates is one of the first instances of that.