They are bearing much of the price or exchange rate risk. Merchants who accept BTC and insta-convert to fiat aren't bearing this risk.
They could otherwise invest in stocks, bonds, real estate, other currencies etc.
Incentivising miners to provide network hash rate
Bitcoin advocates help teach people to avoid the scams (almost all altcoins, forks, ICOs)
Let's not denigrate the contribution of the HODLer