Machines are used to improve yield from ores, such as the macerator. Electro Furnaces don't require any coal to process the ores, which is helpful as well.
Minecraft sorts the world into 'chunks' - 16 x 16 blocks. If a machine is kept running in one of these chunks, it will keep the whole chunk active which can cause servers to lag if there are many people on it (or many machines in different chunks).
The GregTech mod adds a huge amount of content and machine progression to Minecraft. Starting with Bronzen Steam-powered Machinery, players work themselves up to more and more high-tech technology.
GregTech changes up the ore generation so that ores, both vanilla and the plethora of new ones, spawn in major veins rather than random small deposits. While finding these isn't as easy as with vanilla nodes, they make gathering the vast quantities of raw materials GregTech industrialisation needs, much more viable.
While tweaks can be made through the configs files, Gregtech (with many of the changes it makes) is notoriously hard, and advancing through it takes a lot longer than through other mods.