From a 3 page article on SHARE by Bill Lawton of IBM in the Issue 4,
1990 edition of Personal Systems.  Here are some excerpts:

"Share is an extension of DOS that provides 4 major functions:

 o File sharing - SHARE is required is the machine is a PC local area
   network (LAN) file server

 o File locking

 o Diskette change protection

 o Support for File Control Blocks (FCB) file I/O on fixed-disk
   partitions greater than 32 MB.  SHARE is required if any partitions
   are greater than 32 MB."

"A function of SHARE not commonly known is the diskette change
protection.  SHARE does not actually contain the diskette change
logic, but this logic in DOS is inactive (turned off) unless SHARE is
loaded.  This function allows DOS to ensure that users do not remove a
diskette from a disk drive while a file is open on that diskette.
This will prevent the data corruption that could occur without this
protection."

"Some applications do not expect any form of file-sharing rule to be
enforced during file opens, and therefore may not work with SHARE
loaded unless this function is inactive.  This is why DOS loads a
partial function version of SHARE if it detects a 'greater than 32 MB'
partition, and SHARE is not already loaded."

"If any of the inactive functions are needed, one of the other methods
to load SHARE during bootup [i.e. from CONFIG.SYS, from AUTOEXEC.BAT]
must be used; or SHARE can be loaded from the command line."