There are many reasons for trust funds, one of the main reasons for it is it isn't subject to inheritance taxes.
The other reason is many testators opt to do a trust instead of giving out a lump sump payment is simply because giving an 18 year old £500k may see £500 left after a year. You never know what that 18 year old would buy, for all you know he or she would binge it on a trip round the world and a fast car that may end up killing him. Where it is given from a trust, you get to stagger the payments and generally if it is properly invested it can go on for decades while still being beneficial to the beneficiary.
My late maternal grandfather did set up some trust funds for most of his grandchildren and children, I didn't get one because he opted to give me a lump sum before he passed on, together with all his children. Can see the vast difference between those who received a trust and those who didn't, many who got a lump sum had all since squandered it off.
I've also set up 3 trust funds, 1 is an educational trust fund that would provide 2 of my kids for their university tuition fees as I foresee in 15 years time when the eldest one goes to university tuition fees would be at least £25k per year. The other fund is meant to provide the 2 with sufficient living expenses when they go to university. The final trust fund is meant for my wife in case something happens to me, so she could continue to educate our children and also to ensure they grow up without worrying where their next meal will come from.