The short answer
An EWS1 form assesses fire risk related to a building's external walls. Some lenders require it before offering a mortgage on flats.
It exists because cladding and external wall systems became a mortgage-lending issue in their own right. If a building needs one and doesn't have it, that alone can stall or block a mortgage application regardless of anything else about the flat.
Why this matters for your purchase
Building safety concerns like this are exactly what PropM8's building safety section checks for, so a mortgage-blocking issue doesn't surface for the first time at valuation stage.