Comment by dspillett

As well as keeping rows of similar age roughly together, which may create access efficiencies but should not be relied upon, relatively ordered keys massively reduces page splits as the index grows (this can be particularly significant for clustered indexes as implemented by MS SQL and others) because you are always adding items to the end of the index. This can reduce the size of the index overall, potentially a big problem if working in a situation where your data+indexes do not fit comfortably in RAM (you don't want your precious RAM half full of half-empty index pages).

Yes you can shrink it down when things get bad, but if your index is large enough that you care then it is large enough that this reorganisation is going to be costly in time and may slow your system while it happens (or cause a complete lock if your DB doesn't support, or optionally supports and you don't select the option, this as an online operation.