PDStorage can be used to "remember" values across transaction. If you write a value to PDStorage, it basically gets written into a database, which survives across transactions and even process restarts. A good example is maybe a "Welcome Page". If a user accesses the Internet the first time, you show him a "Welcome to the Internet" website, and write a value to PDStorage that this user has seen the site.
The next time a user accesses the Internet, you can check if there is an entry in PDStorage, and based on the result check if you want to display the Welcome Page again or not. Almost any kind of information can be "remembered" this way.
sorry I think I misunderstood the first question. You are right. The PDStorage "database" is kept in memory, to read/write data. Data kept in memory will go on a process restart or crash, so the settings define how often the database is stored to the disk. The second setting defines how often the database will be synched in a cluster (central management). Changes are not synchronized in real time, but following the interval configured here.