In which cases are grids more useful? With grids I mean something like the homepage of youtube, where you have many rows of videos, and in which cases are lists better, such as the youtube search results, where you have only one video per row??
That depends on what you want the user to do with this information.
Lists are usually used to just display line item information so the user can select it to do something with it.
Grids, on the other hand, are usually used more for interacting, to let the user change a cell of information, etc.
The YouTube home page is less a grid and more of a ListView type of display with categories.
The search results are lists because the user is looking for a specific item and that makes it easier to find what they are looking for, while the home page allows the user to just "scan" the page to find anything interesting-- it's not as important that the user digest everything they see on the screen.