My understanding is that butlers were originally the servants in charge of the wine cellar. So why is it that we now think of butlers as general attendants?
It's important to note that there are two different jobs:
Lord Montague's Book of Orders and Rules, 1595 gives an example of an enormous estate that functioned as village-sized command economy. There top position was Steward, while the Butler was in charge of the Buttery (just as the Pantler was in charge of the Pantry). It was a Gentleman of the Chamber who worked a a personal assistant. The Butler kept stock of the wine and ale, and also supervised the waiting on tables.
In contrast 150 years later, Mrs Beeton's Book of Household Management, 1859-61 has shown a shift in the position of Butler which has merged with that of the Steward. The Butler is now the one counting the silver, and announcing dinner. Certainly as the wealth shifted from countryside to city in the 19th century there was not the need for as many servants, and many of the former household products were available commercially.
Mrs Beeton notes that in household with fewer servants, the butler may be expected to to perform some of the functions of the valet. If you could only afford one male servant, you might call him the butler, even if he is doing all the jobs.
By 1922, Emily Post simply states that in smaller houses, the butler is the valet of the head of house.