Simon Thoumire does that in slower tunes: just to colour the character of the sound, especially for longer notes at the end of the line.
For example:
As a more obvious effect in
And in an more delicate way in
(well recognizable in 0:37 - 0:42)
As I just learned in this thread technically this is tremolo, but its musical function is very comparable to vibrato on other instruments; on a violin for instance.