A Will is a “declaration of intention” of the person making it. It takes effect when the person dies. It must be in the form that is prescribed by law i.e. legal in the eyes of the law.
Because it is only a declaration, it mean a Will can be revoked at any time and you are still free to do with your assets any way you choose. For a Will to become an official legal document it must be signed and dated in the presence of two witnesses.
A beneficiary or their spouse cannot be witnesses or they automatically lose their share.
This requirement is the same whether you have your Will prepared by a lawyer, by software, or through online wills services.