I read it as being in order to help the Collinses. If Lizzy married Darcy and thus had some influence with him, she might be able to put in a good word for Mr Collins if ever other livings became available over which Darch had some influence or control - or possibly even high status and more lucritive positions within the Church. Another living with a bigger parsonage house and with a greater income might well have been to their liking :) . I think it was also possible for a clergyman to minister to more than one parish - and so have more than one 'living' - at a time.