There is a purpose to that: in interesting or heated threads, someone who posts something asinine/juicy/etc can go back & change what they wrote, & then the responses make no sense. The "modified" gives later readers a hint that possibly they are not reading the original juicy post.

However, some boards have a window of free modification, usually 5-10 mins, where you can go back and correct grammar or spelling errors without the modified message showing.  That's a nice, user-friendly feature to have.

for the reasons kerri mentioned.  

Other forum software options offer PREVIEW options (preview, save as draft, ...).  

TER Forum software is not so good for what the forums have become.  A discussion of other forum software options came up recently in S&P: "Message Size of each post. link - by impposter, 11/29/2015 3:51:31 PM"