Quickest way to get your head around a fiver is to noodle around on 5th fret....on 4th E string this plays an A whilst on the thickest 5th string (if tuned to low B) it should play an E. So instead of playing low E open on 4th you start using 5th fret on 5th string. For patterns with a long stretch down near nut end it is a whole lot easier shifting up the neck to take advantage of an additional string to access that lower note.
Point in case is main riff in Money by Pink Floyd. Much easier to play on fiver as less of a stretch however when swapping back to a four banger it takes a few moments to remember where those notes are in a different position. Good way to force yourself how to explore many different positions on the neck by swapping back & forth between 4 & 5 strings.
Tend to find the low open B and bit too low and hang out mostly around 5th fret and maybe down to 3rd.
Otherwise it is a good thumb rest as you have already discovered.
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