The program allows to create new partitions or merge (unify) them through a well organized and pretty intuitive interface.
Creating a new partition is recommended when you install Windows on a new computer with one hard disk. This hard disk can be divided in multiple hard drives (C:, E: and so on) and is recommended to create a separate partition where will install operating system (usually C:). In another partitions, such as E:, you can put personal files, documents, movies etc. For example, if computer is virused, in almost all cases only drive where is Windows installed will be affected. If you will need to reinstall Windows, you will not lose personal files from drive E:.
Another use can be to create a new partition from an actual drive. For example, drive C: or drive E: may have too much space, and you can take some free space from here and create a new partition (for example G:) where to keep other files, such as backups or only pictures.
With Easeus Partition Manager or Master you can format hard drives (NTFS, FAT32 ..), change drive letters, resize your existing partitions, merge different partitions into a single one.
When you use this program is recommended to close other applications, because almost all operations requires computer restart.