In some episodes, I've seen Marinette has a backpack.
Which got me wondering, why does she need a purse if she's got pockets and a larger bag to carry things around in?
As far as I can tell, the only thing in the purse is Tikki.
Which would imply it's just a convenient way of carrying Tikki around while keeping her out of sight.
This only works if you don't open it to look out and someone notices you.
Therefore, I am wondering if this is the only reason Marinette has it, it's basically a place for Tikki to live in when they are away from home.