Taking proper legal advice will help to ensure that you consider all  your options and the implications of your decisions.  It will help you to think about issues you may not have thought about or known before.  It will also help you to make decisions which are as tax efficient as possible.
If you have complicated financial or familial arrangements, it is sensible to take advice from a suitably qualified lawyer who specialises in this area of law, as they will take the time to understand your situation.  They can then ensure that everything is considered and addressed correctly.
Taking professional advice will ensure that your will is valid and compliant with statutory requirements.   A will which fails to comply with technical statutory requirements will be invalid.