As discussed in the previous mitzvah, there is an obligation to return a lost object to its rightful owner. But this requires some effort on the part of the finder and maybe you just don’t want to be bothered. Therefore the Torah prohibits us from pretending we don’t see a lost object. Abdicating responsibility is not an option. If you pretend you didn’t see the item, God knows the truth.
The reason is as we said in the previous mitzvah: everybody loses something from time to time. It benefits society to help return things to their rightful owners.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Baba Metzia starting on page 21a and codified in the Mishneh Torah in the eleventh chapter of Hilchos Gezeilah. This mitzvah is #269 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #182 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.