It is easy to keep track of the file, because you only need its starting block and size. It means that space must be pre-allocated for the file before writing it can take place. In a linked list layout, the file is divided into blocks and part of each block is a pointer to the next block. Keeping track of each file is also easy, because you only need the start of each list. Sequential access is easy.
Random access is hard.
Because space is used by the pointer, reads should not be in powers of 2. In an index method, a table is maintained of each block of the disk. Entries in this table for each index are the value of the next index for a file. Then you can store the start of the file somewhere else. This makes it easier to find where a file is, because the index table can be stored in fast memory instead of on disk, and actual data can be full blocks.