The list is endless! But ours will list a few. Common ones that people use them for are:
-breeding fantasy
-bukkake fantasy
-simulating a MMM or MMF threesome
-cum fetish
-lesbian partners to use on their bisexual partners
-for cis women to peg their male partners
People can also use them simply if they're tired and can't 'get it up', or just want to try something different.