well let me give you my case..
i had score of 425 points when i created my profile back in feb 2016. at that time, CRS cut off was around 451, and it kept increasing and at one stage during the year was steady around 490 points... obviously i couldnt wait (actually at that time i didnt even know CRS cut off would fall that much since lowest it ever reached was 451) for scores to fall down, so i opted for OINP... which costed me $1500 (i had to pay that twice, but thats a different story for another day)
when i got nomination in december (after i applied in february) my score increased by 600 points, which is basically guaranteed ITA in next draw. then i formally applied for PR application in january 2017.
the reason why i applied for OINP and paid extra amount is like i said before - i needed a guarantee that i would get an ITA... plus even lets say God forbid i made some mistake in the application and it got rejected, i would still have nomination so i can get ITA in next draw and apply again... to give you an example
in Feb 2016 the CRS score was 451, and i got ITA (if my score was say 452 at that time), in march IRCC rejected my application saying i didnt provide PCC in color, and asked me to go back into pool again... in march the CRS cut off was around 470s... that would mean becoz my score is 452, i would have had to wait for several months praying the score would fall further so i can get direct ITA... if i had nomination, my score would be >1000 so i would get ITA in any draw later on...
OK so if my score lets say as of today is 420, and cut off as per last draw is 433, i would still have to wait patiently, anxiously, not knowing if or when the CRS will drop to 420. also dont forget, for every birthday you have, you lose extra points... so i would either take a chance, hang onto my $1500 and wait for CRS to drop, or else pay $1500 for OINP nomination and guarantee ITA.
long post - but i hope it helped clearing your query lol...