The thing about a decent bike fit is that it could correct some bio mechanical issues as well as set the bike up to suit you. It’s almost a given that the bike is the correct size, but if you consider that every contact point has at least 3 planes of adjustment, it is easy to get it nearly right,but not quite right. For instance, are your cleats in the best position, do your feet need wedges or instep support to help correct knee / leg alignment. Are your cranks causing the knee over extension or compression by being too long or short?
You can do it by reading up and taking videos of yourself, but if you’re spending thousands on a bike, £150 could be money well spent to get the best out of it.
Of course, you would probably be fine with a clued up mate or setting it up using measurements and guides, but a lot of people have odd length legs, tight hamstrings, Achilles issues, bad backs, etc etc, and a good bike fit could help adapt the bike to fit you better.