When I went to get a new set of tires for my 1990 240DL, they told me I need new tie rods. What exactly do they do, and will it cause other problems if I dont replace them?

 

Tie rods are part of the steering of the car.
There is inner tie rods which are from the rack out. The outer tie rods connect to the inners then connect to the Hub assembly.
There is a possibility it could cause a very dangerous situation if they are really bad.
I mean like they could come apart and cause an accident.

 

spot on tech. To the OP if you plan on doing your own tie rods, when your done your going to need a wheel alignment. Unless you mark the outer tie rod ends where you stopped pulling them out.. I usually use white out, then thread them back on. You will also need a tie rod puller