Why tank an outside before rendering?

I've got an external rendering job to chop off and re-render. The existing render has only been on 3 years but is cracking up and boxing all over. The customer has given me the invoice showing the date but in the job description it says;

"Render elevations using a Hey'di K11 tanking compound and finish with a 15mm ready render".

Has anyone ever come across this before? My understanding is that the tanking will be the cause of the render failing.