These instruments started out with LCD displays and were modified to use VFDs towards the end of the 'HP' era. I have an HP 8647A with LCD display, and an Agilent 8648D with VFD display.
They are low cost, basic signal generators. The signal purity (phase noise, residual FM) is not very good, though the 8648 is significantly better, and they exhibit wideband noise on the output. They are best suited to 'in channel' measurements and tasks such as receiver alignment and fault-finding. Also useful for EMC susceptibility testing.
Notoriously, the higher-frequency C & D versions of the 8648 allow you to set an output level above what the instrument can actually deliver above about 2.5 GHz, but do not provide a warning that this is happening.