While a review summarizes the experience, a LP allows to look a player over their shoulder and indirectly experience the game from one perspective in its entirety – if both Let’s Player and viewer have the endurance.
LPs have many styles: non-commented, informational, humorous… And their production quality varies too, be it video, audio or presentation.
Sometimes, LPers will contact game developers to receive permission to create LPs. To many creators of games, LPs are a welcome form of promotion and they will always say yes.
FLARE is a collaborative effort of many artists who agreed to release their art under CC-BY-SA 3.0 and I think that FLARE’s LP policy reflects the intention of the license very well.
A complicated case might be a game which contains art that is under the GPL, which could be interpreted in a way, that requires the resulting video, as well as video project files to be made available under GPL as well.
In theory, any LP could be considered “fair use”. However, for-profit use and use of large portions of a work are often considered as not being “fair use” – for example by YouTube.
For game designers, I consider LPs to be a valuable resource, allowing to look up features or part-experience gameplay, where acquiring, installing and playing the game would be impossible, due to time restrictions.
I recommend looking up games that you have fond memories of or which you always wanted to try but the installation effort was too high on lparchive.org or just YouTube’s search function with “let’s play” in the query.