Re: the only good thing Petunia did for Harry (was a challenge or prompt here)
Petunia taught Harry how to sew / mend clothes with needle and thread, and made him do cloth repair jobs round the house.
When Harry's uniform rips, he realises that some cloth tears cannot be mended with magic, and he is scared to throw his uniform into the separate 'needs repairs' hamper cause he's afraid of repercussions. It happens in 1st year since he doesn't know about the house-elves yet.
Ergo, he asks Hermione to ask her parents to send needle and thread / he wishes for needle and thread for Christmas. Or better yet, instead of a 50 p piece, Petunia sends him a small spool of thread and a needle.
Fast forward, Harry now has a reputation and a small business as person who mends clothes better than the house-elves / being able to do it where the house-elves can't.
The only way Snape can show his support is anonymously sending Harry his orders, and sending Harry books about magically enhanced sewing...