Why bother to have a written tenancy agreement (as opposed to an oral agreement)? The main reasons are: 


a written contract is better than an oral contract, especially if L wants to prove breach and be able to claim financial compensation;


if the tenancy is for the occupation of a dwelling house (at a rent of less than £100,000 pa) it will be an assured shorthold, whether it is a written or oral contract. But, L will not be able to use the s21 procedure to evict T until after the first six months;


any tenancy agreement (whether or not an assured shorthold) created for more than three years must be in writing (s54 LPA 1925). 


Source: Painsmith (http://blog.painsmith.co.uk).