You can find pokemon you already have too

Better stats, abilities, and sometimes they come with better items.

Something I've done in the past is to create what I call a Trap Team. Basically just high level 'mons with skills good moves and skills. Skills like Arena Trap are useful for legendarys that try to escape (like the legendary dogs), False Swipe is good for getting them down to that final sliver of HP, obviously status…