To address the limitations first, contesting is seemingly limited to two seats. According to the Representation of the People Act, passed in 1951, and amended various times:

Notwithstanding anything contained in sub-section (6) or in any other provisions of this Act, a person shall not be nominated as a candidate for election (a) in the case of a general election to the House of the People (whether or not held simultaneously from all Parliamentary constituencies), from more than two Parliamentary constituencies;

As you say, if they win both elections they can only hold one seat, or potentially none, if they don't follow the proper procedure. From the same source:

Election to more than one seat in either House of Parliament or in the House or either House of the legislature of a State. —If a person is elected to more than one seat in either House of Parliament or in the House or either House of the Legislature of a State, then, unless within the prescribed time he resigns all but one of the seats [by writing under his hand addressed to the Speaker or Chairman, as the case may be, or to such other authority or officer as may be prescribed], all the seats shall become vacant.

As for why this law exists, some commentators have suggested that the purpose of the law is as an electoral strategy, to allow parties to increase turnout:

Aditya Mukherjee, professor of contemporary history at New Delhi's Jawaharlal Nehru University, suggested that allowing candidates to contest from two constituencies sometimes helped political parties to “mobilize voters” and “shore up their party’s prestige” during elections.

“It is not unusual for politicians to contest from two seats in order to have a multiplier effect on other constituencies,” Mr. Mukherjee said. “This will attract more people to vote.”

Others say that the law exists to give politicians a backup plan to minimize their chances of losing re-election:

Now the question is, why is it that candidates contest from two constituencies? One reason might be that they are unsure about one seat and to book a place in parliament, they contest from another seat of which they are confident. So one seat is undoubtedly safe to fall back upon, if they lose the unsure one, and if the fortune smiles upon them, they may even win both of them. You can call it a back-up plan.

This writer agrees that it's simply there to let politicians take a risk on one seat while not actually risking anything:

He is right. The provision exists not to a create a more robust and diverse democracy but for the convenience of the politicians. It is their back-up strategy at the expense of the people. It hardly displays any commitment of the candidate towards the electorate he is claiming to so want to serve.

Overall, it seems likely that the purpose of allowing politicians to contest from two places is to provide a safe backup option for them to expand their political power by getting a coveted seat, while not risking their current position.

However, it seems that in reality the language mentioned above was not included to increase the number of seats that politicians could contest from, but rather to limit it. This was due to an amendment made in 1996; the text of the amendment is here. Previously, politicians could contest from more than two seats:

The rule of limiting the candidate to contesting from a maximum of two seats was introduced in 1996 through an amendment to the Representation of the People Act (RP Act) of 1951. Before this law, leaders were allowed to contest from as many seats as they could.

And:

Biju Patnaik, the veteran leader from Odisha, earned the dubious distinction of contesting the most number of seats simultaneously, running for four assembly seats and one Lok Sabha seat in 1971.