The issue is, you buy 1 lot of spanners for the car, then go to do another job with the car and find you don't have the size....So put the car back together and go source the correct size.....This can happen when parts have already been replaced and you find the spanner you used to undo the part doesn't do the part back up again....A good example is the strut top....One brand may have a 24MM nut, whilst the next brand has a 15/16th nut.
The you have the situation of, when you put something on the car, you don't have the right spanner for the job...Then oh dear, off to the local tool shop to find the correct spanner.
8mm is used on the bolt, that adjust the strut at the stub axle....15MM is the rear brake caliper bolt, that flips the top away from the main body....
Do yourself a favour and price up tool kits and save the spon doolies and get the kit....You'll be happier in the long run