Having landscape be modular has advantages on multiple levels. On a personal level a builder can reconfigure and add to their own landscaping set-up without having to rebuild to make changes, useful for adding surroundings to their own MOCs, photo backdrops, etc. The main purpose for a modular standard, though, is that when multiple builders put on a combined display they can all build to a unified set of dimensions allowing them to convene, set up, and tear down a great looking landscape fully integrated into their MOCs with minimal effort.
The second core feature of the Base8 standard is the use of BPB. BPB (Baseplate Plus Brick) is a technique for raising slightly the ground level of your landscape to, as the name suggests, one brick higher than baseplate level (a total of 7 LEGOmetric units). There are several advantages to this, mainly that lakes, rivers and streams can be placed lower than ground level for a much more natural look. It also opens up more opportunities for features and elements that you otherwise might not be able to use in a flat landscape (see here for example).
In order to cover a big area economically in a large display it's usually necessary to use a lot of baseplates as ground, however, so to keep a ground level of one brick across all parts baseplates can be laid atop some 9.6mm (roughly 3/8ths of an inch) thick material such as cardboard, foam-core or plywood or BPB supports built from spare plates and tiles. See below for a number of examples.