Eyelashes are part of the appearance of our eyes, and the human eye is something that gets noticed by others. If we alter our eyelashes we alter out appearance to some extent, though observers tend to notice the effect rather than the actual change.
Of course eyelashes do serves a practical function, perhaps several. The reflex action of closing out eye(s) when our lashes are touched is well known. This obviously provides some protection should we be hit. But lashes also seem to help with airflow and evaporation.
Science has observed that almost all healthy mammals have lashes one third the length of their eye. It seems this is optimal for keeping the eye both moist and reasonably protected from dust and foreign matter. Lashes longer than this one third length offered no real advantage. Straight or curved lashes made little difference; as long as the lashes remained extended from the eyelid they worked well.
As we age our lashes tend to thin out. This varies greatly between individuals. Those with very sparse lashes may find that their eyes suffer some dryness. Lash extensions can be beneficial here. But people mostly extend their lashes for the cosmetic effect.
For a semi permanent eyelash extension talk to 5N2. Whether it is to restore your original look from a youthful past or simply for aesthetic reasons, good quality extensions are low maintenance are highly effective.