When you have no more than 1m zeny, limited supplies, limited knowledge and eden gears, partying is a godsend. You divide tasks, you plan, you get exp faster, it's awesome. I did it a lot when I first started (granted, with friends, but still).
When you have OP gears and can dish crazy damage and/or mob huge number of monsters, plus access to BMx3, parties might slow you down greatly, because you're basically replacing them with these gears.
Partying is more useful and fun in instances/quests or in planned activites with friends/guildies. Of course I can solo OGH, but if I can bring people to chat, have fun and save supplies, why wouldn't I help them?