"езжать" is used:
1. as imperative.
Imperative of "ехать" is highly unstable in Russian.
?Едь sounds strange and is rare.
I disagree with Elena - may be for a modern Russian it sounds acceptable and even old-fashioned, but if you check in books - it is the rarest option! I think, in times of classic Russian literature it was, conversely 'folkish', not 'bookish'.
?Ехай can be met in dialects. Also in children speech. Sounds highly... dialectal/colloquial. Its users are likely to be laughed upon by snobs.
езжай - is a comporimise form. It still sounds somewhat colloquial.
поезжай - more common and somewhat more literary.
Often, to avoid "colloquial" shades we just don't use imperative and say it diffrently, like "I advise you to go..."
2. in imperfective of prefixied forms:
поехать - ?поезжать (rare unless as an imperative)
проехать - проезжать
наехать - наезжать
выехать - выезжать
объехать - объезжать
3. there is an additional (to those mentioned in your textbook:-)) verbal tense in Russian:)
"езжать", also "езживать" can be used similarly to "бывать".
I hesitate to define its semantics, because in modern Russian this form is getting less common (though бывать is common).
May be in past it was just used for an action repeated many times. I'm not sure what they had in minds in 19 century:(
In modern Russian it is stylistically marked, sometimes as a play.
An educated Standard (!!!) Russian speaker would use it semi-jokingly, to imitate 'folkish' and archaic style....
... OR, if he loves to read books (these forms are common in classic literatrue) - just to use the full richness of language, without jokes.
Mostly it is used to mean:
you had a habit of doing this in past, it was common and customary to do this.
Он часто езжал к ним.
Also to emphasize that you have experience of doing this (and more than once likely).
Бывал я в Стамбуле.
Видал я и не такое.
Я такого отродясь не видывал!