A couple different reasons:

1)  The principal of the matter, now that we have become serious, bonded with each other's children, etc.

2)  An attorney might not be in the budget for years at best.  I have 3 children, she has 2 (one from her ex, another from an ex boyfriend prior to that), and she only very rarely gets child support (yet refuses to do anything about it to keep the peace).  We have a very tight budget, even though we both make a modest income. 

3)  We are hoping to be married in the not too distant future.  We love each other in a way neither of us even knew was possible.  Although now, with how much of a hassle this is turning into, we might just do a ceremony, but forego the legality of it.  Regardless, we don't want to do any of that until the divorce is finalized.

4)  Her ex is a liability.  She has lots of assets she worked hard to earn.  He jumps from job to job, and oftentimes paycheck to paycheck.  Very, very few assets on his part, if any.  Hence why she rarely ever gets child support.  But again, she refuses to do anything about that to keep the peace.  With that said, she is concerned that either he could end up running into some financial issue that might screw her, or since his wife doesn't particularly like her, might encourage him to try to take advantage of splitting assets and acquiring a bunch of the stuff she earned on her own.  Right now, that's not a problem, but she does worry that if he's desperate enough and she encourages him enough, it could happen later on.  So, once she gets this done, that concern is gone as well.

There are other reasons too, but those are the ones off the top of my head.  She really just wants to cut that tie to him as well.  In her eyes, it's a symbolic respect thing for me that she really wants to do, if that makes sense.