The short answer
Many risks are only identified during the lender's valuation, which typically happens weeks into the process.
By the time a mortgage valuation flags a problem, you may already have paid for a survey and legal work, and put weeks into the purchase. Getting ahead of the same risk factors earlier avoids relying on the valuation to be the first line of defence.
Why this matters for your purchase
Getting a PropM8 report's building safety and environmental checks before you apply means you're not relying on the lender's valuation to be the first time these risks are identified.