Soloing mobs or seeking a party was the previous method for earning experience points. Both had their drawbacks.
Soloing experience points would take too long to earn a few hundred FFXI Gil or thousand experience points; needles to say it's very risky because if you died, you're right back at square one because of the experience point penalty for deaths.
Seeking a party had its drawbacks too. Mainly because you sometimes could waste so much time just trying to get invited to join a party or for the go-getter; you could start building your own party. Although; building your own party seems to be the smarter choice because you could get whatever jobs you felt would be appropriate, you're still taking a risk of having bad players in the party. Even with FFXI Gil just one terrible player in the mix, your experience party could be a dud and earn experience points at a very slow pace. Now ideally if you're building a party you would try to get players you actually knew or linkshell mates to join. This way you're going to have more reliable players in the party and you can almost guarantee it will be a pleasant experience for all.
There are two main reasons FFXI Gil to introduce leveling syncing. The first one was for the current FFXI player that wanted to introduce the game to the friend but couldn't level with them because of the great level gap and it was for the player that wished to level with linkshell mates but once again could not because of the level gap.
The second one was that level sync parties have now become the primary method for gaining points and leveling one's character. Level sync FFXI Gil significantly outweighs the previous experience point methods because it offers a much more enjoyable scenario for the entire party. Gone are the days of seeking a party for hours on end for most non-desirable jobs. Even the once thought solo jobs can get in on the action.