There are a number of reasons to do this. The most common is to encourage action. Many people will play much more loosely in straddled pots (which is to some extent correct). Thus gamblers like it. So do tight players who end up with a hand that time!

Another is, quite simply, to get more money into the pot. If you have a significant edge on the table, you want that money in the pot, because you can outplay them for a bigger pot further in the hand. Two hands as example:
1 - no straddle: 2 limpers to Cutoff, who raises 5bb. Folds back to limper #2, who calls. Pot is now .5bb (SB) + 1bb (BB) +1bb (folding limper) + 6bb (calling limper) + 6bb (player) = 14.5bb. The flop comes, limper check/calls the 10bb cbet. Pot is now 34.5bb. Then limper folds to the turn bet. CU made 18.5bb (minus rake).

2 - with straddle: 2 limpers to Cutoff, who raises 10bb. Folds back to limper #2, who calls. Pot is now .5bb (SB) + 1bb (BB) +2bb (UTG/str)+2bb (folding limper) + 12bb (calling limper) + 12bb (player) = 29.5bb. The flop comes, limper check/calls the 25bb cbet. Pot is now 79.5bb. Then limper folds to the turn bet. CU made 42.5bb (minus rake).

If you can get everyone to do this, it should increase your profits significantly, assuming your edge is there. Even if only the UTG/str player does it, if that player has enough of an edge, this could pay off.

Finally, many people play at stakes that make them comfortable. If, however, you force them to play higher stakes, they are going to be less comfortable. That ultimately means they are likely to play less optimally, which ultimately means more profit for you.