Well, it's a bit more than just breaking it up isn't it, andd you need to buy quite a lot to replicate the amount in a tin and possibly because even in sunny Saundersfoot, Roma or San Marzano tomatos may not be available all year and the canning process provides the perfect tomatos for the job? And of course they live in the cupboard until you need them unlike fresh ones. With some things canned is better than fresh - like frozen peas are better than shop bought fresh.

They also have more flavour than most of the tomatos that are in most UK shops most of the year round.

So I'd say the answer is availibility, better quality, ease, convenience, and taste?