MQLs are an internal alignment construct. Marketing needs a definition of 'we did our job, sales now takes over'; sales needs a definition of 'we are not going to chase every random form fill'. The MQL is the contract between the two functions.
A useful MQL definition is specific: title, company size, behavior, source. Vague definitions (e.g. 'anyone who downloaded an ebook') generate noise that sales eventually ignores, breaking the contract.
The MQL → SQL conversion rate (typically 15–35% in B2B) is the most important diagnostic. A conversion rate above 50% suggests the MQL bar is set too high (you are passing only obvious wins); below 15% suggests the bar is too low (sales is wasting time on unqualified leads).