While the responses below address the advantages to having a will with respect to finances, inheritance, etc., I think for parents with minor children, probably the most important reason to have a will is address custody of the children.
If you are a single parent, or even if you are not, there is always the possibility that your children could suddenly be left parentless. Establishing in advance who would retain custody following such an unfortunate circumstance could be of critical importance in making an already difficult transition as smooth as it could be.
Without a will, state probate law could place your children with relatives who you would rather not place them with, or such relatives may not really be able or willing to accept the custody of the children. Or maybe you would really prefer that someone other than a relative would take custody. In this case, a will would be an absolute necessity. In addition, you might also want to vest custodial control over finances to someone different than the person who has physical custody.
These are not really pleasant things to think about, but it really is in your kids' best interests for you to think about them and to write down exactly what you want to happen should the worst case scenario occur.