Never been approaching a complex, busy junction and had a doubt about your lane position? I know I have, and it's nice to have the sign being shown permanently on your screen, well in advance, with the lane you need highlighted. Doesn't mean I'm not looking at physical signs and markings, but they might not be immediately visible for a quick double check, and by the time they are, you might have lost the opportunity to switch lanes safely. Of course no one needs it, but taking that to it's logical conclusion, then no one really needs a sat nav at all - it's just a handy tool.
I'd go as far as to say that lane guidance is probably the most useful aspect of any sat nav system - usually, I know the general route I'm taking so the only time I'll actually look at it is when approaching a junction or interchange I don't know. - it's nice to have what is essentially just the junction signage repeated and on permanent display in the car. Also handy when driving abroad where signage or markings aren't as clear and you might be less familiar with destinations or major route designations, or traffic is a bit more hectic and a lot of your concentration is on making sure some impatient local doesn't take your side out (**** you, Peripherique).