PT (letter of interest) is not a certificate, but only a letter invite you to apply OINP.
1) Once you receive PT, then you apply OINP, after approved, you will receive nomination certificate and 600 points in EE pool. It could guarantee that you receive the ITA in the next draw.
2) If your score is high enough to receive ITA in EE, it saves your money and time in OINP for the 600 points.
3) Even PT valid for 6 months, there is still very very high chance that it will reach cap soon (it happened last year). Then your PT sounds like valid, but you can not apply anymore this year. After six month, it became invalid and you have to wait for new invitation. Ontario only have 6000 for all streams, once it reach limit. Ontario has to stop all application and wait for new allocation from federal government in 2018.