A Single Asset Hive offers the possibility to deposit certain coins and receive interest in this coin. So you increase your total balance of this coin. Such an investment should be made (only) if you are convinced of the value of the coin.
Single Asset Hives are also considered to be very secure but usually don't pay out as much rewards as Stablecoin Hives.
Obviously, you are subject to the risk of price changes of the underlying coin.