Suspension Strut

how do you know when the suspension strut on the X5 needs replacing, and just what does the strut do????

I have 128,000 miles on my 2003 with 3.0i motor and have not changed the front struts. Do you have air suspension? Usually the lower ball joints and control arm bushings become worn. If you don't see oil leaking or air leak then I would not replace.