This is a requirement to make the settlement agreement valid. In order to be a valid settlement agreement you need to have independent legal advice from a solicitor or other person who meets the legal definition of a ‘relevant adviser’.
The rationale for having to seek independent advice is probably because the Government has given you lots of rights and it wants to avoid you being pressured by an employer to sign a piece of paper for an insignificant sum to avoid those rights.
A settlement agreement is a contract but this additional protection is designed to protect you so you understand what you are signing and if we provide advice whether it is a good deal for you.