Reasons to mode switch:
- Fuel Rat client in Private Group or Solo, need them to swap for rats to instance with them (quite common reason)
- Fuel Rat themselves in PG/Solo, need to swap to Open to instance with client
- in Open/Solo, friend wants to play in PG
- in Medium ship at outpost, NPC stuck on pad. Swap modes to actually land
- in Med/Large ship and AFKer/griefer pad-blocking. Swap to land
- exploring / pootling about in Open/PG, find pretty X. Swap to Solo for high resolution pics
Probably others, but I haven't had a coffee yet...
Solo is just a PG of one, so maybe they could in theory merge those mission boards together.
I've found Solo mode very useful personally, due to the lower network requirements. On holiday in Italy recently, in the mountains, and Open would just crash anytime another CMDR instanced with me, and at least half the time loading the mission board. In Solo it was much improved (warp tunnels would often take a while still).