Some people just can't help it, they need their favourite legendaries in their team so as soon as they catch them, they start training them. But others just catch them to fill the Pokédex and leave them in a box.
Personally, I catch them and leave them in a box. I have to say that before DPPt, I often used them in my team and leveled them up to 100 but now, I see them as rare and valuable Pokémon (in fact it's what legendaries are) so I just keep them untouched (I do the same with Pokémon events, even if they're ordinary Pokémon like a Magmortar).
So what do you often do?