Tires are the only part of your car that touches the road, and they tend to go off slowly enough that you do not notice.
An inspection measures tread depth at several points across each tire, looks for cuts, bulges, cracking and embedded objects, checks the pressures, and reads the wear pattern. That pattern is genuinely useful information: wear down the middle usually means overinflation, wear on both edges means underinflation, and wear on one edge often points to alignment.
You get a straight answer about how much life is left and what, if anything, needs doing.