The reason why I do ambu:
1) pretty much the only group content in game that still gives meaningful rewards. 80% of good gears these days all came from Gil Gil and more Gil.
Rema: Gil. Hq: Gil. +3:Gil. Basically without gil nothing gets geared.
Ambu is a very casual Friendly gil farming method because it's more efficient than solo, as a busy person with life I prefer to make Gil spamming VD for 2hr, instead of solo salvage for 10hr. I play on small server so queue line is extremely low that it's possible to do multiple VD in an hour, which is nice.
Ambu is also less mental work than make Gil via merc. Don't need to look for buyers nor deal with people drama that comes with merc.
2) its fun: most content these days are alliance content and its zzzzz fest, seriously. Other contents are too easy. Dyna: lag fest zzzzz. Omen:afk leech cards. SR:zzz and ***never drop. Master trials: nobody wants to do them again after they got glowy....
By comparison there are always strategy to try in ambuscade, or things to optimize. Many niche sets never get a chance to be used in other content, but ambu gave them a meaning.
Things like, cor/whm main healing, crit rate sets, white dmg dps sets, aoeing, cor enmity+ set etc etc. There are absolutely no incentive to use those in other content, but in ambuscade it's possible to use those, for reasons.
We are in an era that people can optimize a job endlessly, may as well do content that could use those 123456789 sets.
3) friends/connection building. The best way to make friends in ffxi is do stuff together. I prefer to use my time in group content instead of solo cp or Omen cards. Solo is.... Horrible.
4) schedule: it's something we can enter anytime. I don't have consistent play schedule so it's tough for me to do other content that needs to match ls play schedule. I can just log in and do ambu when I play.
Other contents are horribly boring anyways. Especially dyna lag fest. It's *** painful. With limited good content choices ambu became best choice.