Of course you'll be diluted as the company grows, so the point of asking is not to calculate exactly what your payout will be. But you can use the fully diluted # to estimate your ownership based on average/expected dilution from the current stage to IPO/liquidity.

I.e. suppose a startup grants you 20,000 shares. There is a big difference between the startup having 1M fully diluted shares (2%), 100M (0.02%), and 100B (0.00002%). In the first two cases you're getting a reasonable share of the startup (depending on the stage); in the last case you are likely getting scammed.