There is only one type of file access that was support by the early OS which is called as sequential access.
In sequential access file systems, a process could read all bytes/records in a file in order, starting at the beginning, but couldn't skip around and read them out of the order.
Sequential files could be read as often as needed.
Sequential files are very convenient when magnetic tape was the storage medium.
But when the disks as the storage medium came into picture/use for storing the required files, then it became possible to read the bytes or records of a file out of the order, or to access any record by the key, rather than by position.
That files whose bytes/records can be read in any order are called random access files.
Random access files are required and essential for many computer application, for eg., database systems.