Simply because dividend income at the hands of investor is tax exempt. Therefore, by making that short-lived deal, they get a tax break for one whole financial year. You get 5,000 units. Of course, you lose Rs 5 per unit when you sell at this price, but don't forget that you have already received a dividend of Rs 5 per unit. One, it throws up a loss that you can use to offset other capital gain and two, it gives you a Rs 5 component in your earnings that is absolutely tax free.
Some funds abet the process because they gain on two counts.
One, there is a massive inflow of funds around the time that dividend is declared. And two, it helps them earn large interest income by parking the funds in short-term instruments. The reason why big-ticket investors do it is the huge notional loss they are able to book against their capital gains. The figure could run into several lakhs or a few crore of rupees. In your case, it would be a few thousands of rupees.