"sometimes" it will id an app that nmap does not (but not that often for me)... the amapcrap tool that is bundled with it can also sometimes help you identify an application running on a non-standard port but honestly it doesnt come up that often. 

i'll also add that nmap (if you dont use the -sV option) will just id an app but the service port, say 1433 for mssql because it runs on that port, but if you run something else on that port say your webserver or something like that nmap may ID that correctly or it may ID it as mssql.  amap will correctly id it as an http device but will probably not tell you if its IIS or Apache or whatever...