Portable mixers: why would we need one?

["davimon100" davimon100]

This is going to appear an ignorant question to anyone for whom this is obv=
ious, but if you already have a multi-channel recorder, with quiet mic-pres=
, gain and sensitivity controls, under which circumstances would one need a=
 mixer as well?  In the main we try to keep the recording bags as light as =
possible, but there must be some advantage to lugging around a mixer, or pe=
ople wouldn't do it (and I know some people do).

I imagine there has to be some benefit to letting a mixer mix and a recorde=
r record but if a recorder already carries some of the features of a mixer =
... then why?  I see a lot of used mixers for sale; do people grow out of t=
hem or do their recorders do enough of the mixing not to justify owning a s=
eparate?

Thanks.