You are funding an annuity. Know what that means. But funding an annuity takes time. It builds. For most, that annuity is funded over time and funding for it accumulates over a... yes... period. This process happens in the form of accumulation units, which are bought in stages over time. God this is dull.
Note additionally that over long periods of time, markets go up and generally speaking, the more aggressively you save during the accumulation period or phase, the more money you'll have when you want to play golf and show up for the blue plate special dinners at 4:59 pm.