Hi,
I have 2 ten pole allotments and still cannot fit everything I want into the area that I have. Therefore I grow things for four reasons as follows:-
1) I grow things that are expensive to buy such as asparagus, soft fruit, salad crops, cut flowers etc.
2)I grow things that are difficult to buy really fresh and tasty such as sweetcorn and new potatoes.
3)I grow things that are impossible to find in the shops such as pea tips, different types of tomato and peppers, celeriac and sweet fennel.
4)I only grow things that I like.
If you plant sensibly, for example I have 10 blueberry bushes that crop early, mid season and late season, then you wont get huge gluts. Fruit also has the benefit of making large swathes of an allotment quite low maintenance.
Cheers
David