What is the purpose of suspension lockout? I'm trying to decide which 29er bike I want to purchase and one of them has lockout. I know what lockout does but I'm not sure what it is used for and if I will need it.

Thanks

Update: We're good now. It's been 6 years (Holy crap) since I've posted this. I'm definitely fully aware of lockout. I've also realized my Rockshox Recon Silver lockout is actually 100% lockout since there's a buffer.
You turn it on when riding on smooth surfaces or going up hill to improve pedal efficiency.  It's hard to know if you will need it in advance because without riding the bike you won't know how well the suspension design handles pedal bob (the energy lost by the bike suspension compressing under pedal forces), try and get a test ride.