My Saab has one, apart from abit of fun it serves not purpose whatsoever. Guess you would notice lower boost reading as time goes by and the turbo/engine wears.
....Is your Saab's gauge like the Golf GT and without numeric values?
By giving you approximate but not extremely accurate numerical readings, a psi gauge will give you much more clue about what is happening and therefore encourage you to check out a sudden difference more thoroughly via data logging or similar. Obviously if you have a more major boost loss you should feel it when driving.
Having experienced a boost leak, though a minor one, I'm glad I've got mine and that it displays numeric values.