End-game questions

[EveK]

Tomestones allow people to accrue tokens over time to turn in for whatever kind of gear they want rather than having to get lucky and get the right drop (which is also an option). They primarily buy equipment for levels 50 and 60 but can also be used to buy things like minions and orchestrion rolls. You can technically earn some at any level (via Rank A and S hunt marks) and more as your dungeons approach level 50, but they're intended to be max level currency and the easiest way to get the 50 ones is from level 50 stuff (and the 60 ones from 60 stuff).

The highest item level equipment now is also technically 245 weapons (the highest weapons weapons in existence have historically tended to be 5 item levels above all the other equipment which does indeed presently max at 240 and of course requires level 60).

Rest is pretty much already answered. There's generally plenty of incentive to do what's the most fun for you.