The first and most obvious benefits are in productivity. Time savings are achieved directly by enabling formulae to be more quickly, but equally significantly, by reducing errors in entering formulae, and automatically correcting errors when they are made. Errors in complex spreadsheets can be difficult to identify, and can have multiple consequences as they lead to further errors in dependent calculations.
This leads to the second type of benefit: a reduction in errors caused by mistyping. The consequences produce two types of errant results. Errors may produce values which are not credible, where the values are clearly impossible: 45 June 1997 for example. Alternatively, the resulting value may be perfectly credible, but equally incorrect. These are much harder to spot, so anything which reduces the chance of them occurring in the first place is welcome.
So just for once, laziness can be the new productivity: Let the computer take the strain, and Excel write (and check) your formulae for you.