why do we want politicians making all our laws?
They can't.
They don't know enough about anything. We need outside experts having an input, and this is how we get them.
the house of commons sends a rough first draft of new legislation to the Lords
The MPs in the house of commons know what they want ideologically, but not how anything actually works in practice.
The life peers in the Lords, of which there are thousands, will include many experts in the matter under discussion, whether it be tanning leather, generating electricity from tidal turbines, education of children with Down's - what ever it is, there will be people in the Lords who are experts.
They amend the bill and send it back to the Commons for further discussion.
They do this twice before anything becomes law.
Without them, there would be no expert scrutiny over any bill from the commons