[AYBABTME]

Lower tax bill, as you need to pay taxes on assumed gain when you exercise your options, based on the latest 409a valuation of each share. The lowest such valuation in a startup is usually early in its life, and during your tenure, the valuation should go up steadily (or something's horribly wrong with the company's business) and thus your cost to exercise increases over time.

Another reason is more practical: it's much easier to exercise your options when the tax bill is lower and you just got a job/while you're employed (i.e. you're more plush with money), than when you're quitting/been fired and have 90 days to figure out where to get money from. Basically, it's better to be in control of when you buy, than being forced to buy or lose out.