You may not be able to travel faster than 70mph, but you can certainly use the acceleration that higher power engines gives you. Technically, there is no point owning anything other than a 1100cc Metro, which has 5 seats, a boot and can easily travel at the motorway speed limit. Yet you choose to own a 4WD, 3 litre vehicle - presumably just 'because you want to'. And there's nothing wrong with that at all, be boring if we were all the same.
If you rationally analyse almost anything people do apart from basic living essentials, you can say 'that's silly'.
Personally, I agree with you that it's not necessary, but there are other factors at play. People like taking on challenges, pushing themselves and their skills etc just for the fun of it, I think that this is just part of that.