Why do people who have money buy gold reserves? So they can sell it for more later.
A corps members make money which goes into the corp wallet. It is then invested in plex so that wealth controlled by the corp grows (or at least doesn’t devalue from inflation). Then when that wealth is needed again, say wartime comes and srp requests start rolling or the armoury needs filling, sell the plex for liquid isk.
Isk sitting in your wallet isn’t ‘working’ and will likely lose value.