After beating the 86th level in Single Player mode, I unlocked the ??? and in it's place was the word Endless. I started on level 21, continued onto 41, and then 32. I have not gotten all of the rucksack items (the last one I got was fire stone).

So, outside of throwing random levels at you, what does this mode do?

Nothing at all.. it randomly selects any number of levels for you.
You cannot get the Rucksack items anymore that I am aware of.

to get a Rucksack item you must complete (3or 5 levels in a row IIRC) without getting the Game Over screen. if you continue it starts over and you have to get that many in a row again.. so if you continue a lot, you will miss out on the items.

The items do nothing so there is very little points in it at all other than bragging rights (or obsessive resetting from last save point)