Of the two, PO is what's really important, but PI is of value to know.
PO indicates a read error; data is actually unrecoverable.
PI comes in two flavors, PI error (PIE) and PI failure (PIF) on DVD media. These represent the first and second lines of detection. The structure of this information is different between + and - media. I've read that - media has a smaller set of cross check information, which means a direct comparison of PIE and PIF between these two media isn't very meaningful.
If you simply perform a file by file comparison after a burn, such as the "verify" option in Nero, you know the files came back to you at least once. That's fair enough for data.
If you're playing a DVD movie on a standard player, though, the PIE and PIF scan will give you a hint (not a direct assesment mind you) of the general quality of the burn. Any PO is an error, and in the case of data means you've lost a file. A PO during playback of a movie, though, may result in a momentary skip in the playback, but that's usually less imposing than the wrinkle on a VHS tape (because a wrinkle slides by much more slowly).
A scan with high PIE and PIF, though, even if PO never appears in the scan, can give some players more problems than others. Players have limited tolerance for error correction because playback is a realtime performance. Data discs might be more forgiving, because the DVD reader can take what time it needs to recover the read flaw.
You'll discover, too, that two scans on the same media made within minutes will have different results! This may seem counterintuitive at first. The mechanics behind this are too much to go into, but basically this means the scan is a rough indicator of a single scan of the media, not a direct analysis of each flaw. What one reader considers an error or a failure might not be consider so by another, and even a subsequent scan by the same reader might do better on the second try.
Cheap media usually shows high numbers of PIF and PIE, with widely erratic variations over the surface of the disk (and so does damaged media). This can be valuable to know if you've just cut a disc, and have limited time to cut another if the first is bad. That's a good deal for 15 minutes effort, compared to playback of a 2 hour movie.
It will not tell you if the data in the disc is readable. If there are no PO failures, chances are high that data is fine. Conversely, one or two PO may not mean your data is lost - but it may be time to make another copy.
Scans are a valuable tool to asses the quality of a new purchase of media, determining, for example, if you've purchased a batch of fake TY media, or of the shipping snafu actually did damage the cake in an otherwise invisible way.
It's even more useful to track the "health" of an RW you've used repeatedly, or to see if a dropped disc has been scratched beyond readability.
So, do you NEED it? Well, when you get used to it, it might seem so.