From my limited experience of looking at the history of wills, an annuity was most often a form of legacy.

They may originally have been paid out of the income of the main beneficiary but by mid/late C19 a capital sum was set aside to produce a specific sum annually for the annuitant for her/his life with the capital sum and any surplus income reverting to the main beneficiary.

Now, as Roger says, the annuity is bought as an insurance-type package with no capital sum to return to the estate.

The most likely recipients fall into two groups - the stereotypical distant cousin or elderly maiden aunt for whom the testator felt some responsibility but whose needs and therefore income were expected to be modest or a retired senior servant who had been with the family for decades. There are cases of someone less close to the family, perhaps a soldier who had impressed a military man or saved his life in battle.

There are tales aplenty - fiction and anecdote - of people being expected to live on a truly miserly sum and to be grateful for it, too!