Similar reasons here - we wanted to be sure that my laddie would be brought up by the couple we wanted if we both keeled over before he got to 18.
If we had keeled over the money would have gone into a trust and we left a wee document with the will highlighting what type of things we thought suitable to be paid out of the trust by the trustees on his behalf (extra costs for folk taking him in, university costs, world travel before it became known as a gap year, etc.) before what was left would be paid out to him at 21. All a bit mature for a daftie like me! Definitely worth thinking of doing if you have weans.