I've read that GNev has said Moor Lane is fine for their current needs, but they will outgrow it. The opportunity to jointly take over the AJ Bell is only available now, otherwise Sale Sharks will take it on outright.
I think the rumoured £15m cost to buy it (and so £7.5m for their 50% share) is considerably less than it would cost to build a similar sized stadium, so its a business decision.
In addition, the hospitality options on matchdays and, conferencing facilities on non-matchday is a source of revenue they don't currently have (or not to that extent).