If you have more than one radio and you wish to check them both, typically prior to an IFR flight, the first call will be a "radio check", i.e its purpose is just to check the radio. You can then change to the second radio and call for taxi or whatever you require.
The purpose of stating your frequency when you call is primarily aimed at situations where the ground operator monitors more than one frequency. If you don't say which one you are calling on then how do they know which one to reply to. This occurs if you use HF!
Most VFR fligts will probably only use one radio and there will only be one frequency, so these things are not always necessary, but when they are, you will know what to do. Similarly, if you don't have ATC you will not need a departure clearance. When you go to a field with ATC you may be given a departure clearance which is not a clearance to take-off.
Jeremy Pratt's book is meant to be a training publication, CAP 413 the latest update of which is due out today, is a reference book of recommended procedures.