Their resistance varies over a wide range, many decades, from light to dark.
Their resistance is in a 'nice' range for detecting with meters, and the analogue inputs of Arduinos and PICs, so are very easy for a noob to use.
They are good linear resistors, so are useful in audio volume controls and for controlling the frequency of RC filters, you can't use photo diodes for audio.
They are slow (at least compared to photo diodes), so they are OK for indicating light levels, but not OK for receiving optical communication.
They are strongly temperature dependent, so they are not OK for stable repeatable precision light level measurement.
Having said they are slow, they have a very, very slow 'tail' when increasing their resistance in the dark. Note the data sheet specifies the dark resistance after 10 seconds!