I have frequently seen this used when the user is working in a country other than the US with a non-us keyboard.
On many non-us keyboards, the configuration is VERY different and the EQUAL sign is not in a normal location and on some, not even present at all.
A little know oddidity of Excel is that if you start typing a formula with a plus sign, (omitting the equal sign), Excel will automatically add the equal sign.
I have dug out an old comment from a Polish SE (working a Czech case) I received a long time ago when I inquired of him, why all the formulas started with +
"The “+” is very helpful with Czech, and potentially many other language, keyboards. The advantage is as you write – it has no impact to the result. But when you write formulas in Excel, you often need special characters present only on the EN keyboard. Unfortunately the “=” sign on the Czech keyboard is not on the same place, as with the US-keyboard. If you try to accustom writing formulas beginning with “=”, you would have many errors depending on actually switched keyboard. On the contrary the “+” sign is present on the numeric part of the standard keyboards, and is highly international, and when you start with “+”, Excel would automatically add “=”."