Reasons to Straddle
I know ACR allows straddling. Don't know about other online games. But there are a few times I straddle.
First, if the table is really tight you can use straddling to open it up some. When people see someone throwing chips around it can make them want to do the same. They won't start straddling, but they may start playing a little looser, at least on your straddle
hands.
Second, if you are a tight player but are trying to convince the table you are loose to develop a false table image straddling gets the job done. When someone straddles other people assume they are just here to gamble and it can send a lot of action your way.
Third, if you just want to piss off the BB and SB.
Fourth, if you straddle and you only get callers, raising the straddle can be a very effective way of taking the pot down preflop.
Finally, if you just want to have some fun and gamble while giving yourself a slight advantage pre-flop.
They're not all good reasons, but there are reasons.