What do { } brackets mean when they encompass a function?

The following function, which works when the spreadsheet is opened, is in a
spreadsheet I inherited. If you arrow onto the cell containing this function
it shows up just as shown below:

{=SUM(IF(H$8:H$27="+",$F$8:$F$27,0))}

once the cell containing the function is doubleclicked the, the function
then looks like: =SUM(IF(H$8:H$27="+",$F$8:$F$27,0))
and when 'enter' is pressed the cell reads: #VALUE!

If the same cell is double-clicked to go into edit mode and the brackets are
added at the front and rear of the function, just as shown above, after
'enter' is pressed the cell reads: =SUM(IF(H$8:H$27="-",$F$8:$F$27,0))

I've used various functions quite a bit, but have never seen this before.