As already mentioned, the combination of these two programs allows you to experience the content of Myst Online: Uru Live again offline in a fully playable way, meaning that all the puzzles that require multiple players to solve are feasible in solo. This mainly means Eder Delin and Eder Tsogal, but also Ahnonay, because its puzzle has been modified for the MOULa version.
Drizzle is a collection of tools for the Uru engine, but its main purpose it to convert the MOULa ages into a format compatible with an offline Uru installation. This means, of course, that you have to have the actual files, so you need to at least install the online version beforehand. Additionally, if you have Myst V installed, you can also convert those ages and have them accessible from within Uru as well, since the linking book that leads to them is technically located within the Great Shaft. This doesn’t really serve much purpose, as their puzzles are inactive, and if you own Myst V, you can explore them whenever you like anyway, but the option is there, if you want it.
Once the ages are converted, you need to install the Offline KI (also done via Drizzle) to be able to actually access them. The program upgrades your ingame KI with a lot of commands and features, including a flymode, for those that are curious about how the ages are actually constructed. This allows your avatar to pass through any and all surfaces (walls, ceilings, floors) and objects, and can make for some interesting screenshots. The Offline KI can also make your life easier in several ways, such as, for instance, allowing you to collect all sparklies for the Relto ‘calendar’ in one go, rather than having to wait one year to get them. It can also force the urwin, panuhdoy and sandscrit to appear in Negilahn and Payiferen, respectively.
All in all, if you’re not intending to play online, but still want to experience as much of the Uru content as possible, Drizzle is the way to go.