MPs are your elected representatives - they must be accountable to the people and they must act in the public interest. Write to an MP if you want to petition or lobby them to assist you with a problem (no private disputes and only issues to do with government services), offer suggestions or support a campaign you feel strongly about.
Before you contact an MP, note that their ability to assist is limited. Often the right person to contact will be a local councillor, the Presidential Hotline, National Anti-Corruption Hotline, or the Public Protector.