I would like the ability to search for sets that contain a specific collection of parts. You can now search for a partnumber and see what sets contain that one part, but if you need a specific combination of expensive parts, it sometimes is cheaper to buy a (small) set containing those parts.
For example a suspension/steering-combination of:
2x #6571 steering arms
4x #6572 knuckle arms
2x 76138 shock absorbers
1x #6574 small gear rack
cost about €20,- on Bricklink.
In that case the set 8048 Nitro Buggy - which contains all those parts - and costs about €20 second hand, is then relatively cheaper because you also get a lot of extra parts "for free".