Marmeladov is a "don't let this happen to you" kind of character. His drinking strongly contributes to huge suffering for his entire family. His daughter Sonia has been forced into prostitution and he uses her hard-earned money to buy booze. He claims to love his family, but can't get his drinking habit under control.
It's a little surprising that Raskolnikov seems to care about him so much and to treat him so gently. Perhaps it's because his suffering and helplessness is so apparent. As we know, Raskolnikov has a soft spot for stuff like that.
In any case, if Raskolnikov hadn't helped Marmeladov, he might never have met Sonia. Furthermore, Marmeladov brings out something in Raskolnikov that we don't see often: real tenderness and compassion. Raskolnikov actually acts on those feelings in a positive way and tries to help Marmeladov. Check this out:
When Raskolnikov leaves Marmeladov's house after Marmeladov's death, he meets Nikodim Fomitch, who notices that Raskolnikov is "spattered with blood." Raskolnikov says, "with a peculiar air, 'Yes... I'm covered with blood'" (2.7.92).
This creates a sense of doubling, or even déjà vu, and recalls the murders. This is the kind of thing that makes Raskolnikov so confused. On the one hand, he sees that there is a huge difference in getting blood on your clothes from killing someone, and getting blood on your clothes from trying to save someone. Yet, his mind can't quite grasp why there is such a different.
Anyhow, Marmeladov challenges us. If it wasn't for the fact that Raskolnikov liked him so much, we'd be tempted to call Marmeladov a bad guy. As much as the novel disapproves of alcohol, it seems to consider alcoholism a genuine illness and, as such, places Marmeladov in opposition to people like Svidrigaïlov and Luzhin who appear to get off on being mean and abusing their power.