You don’t always need to have your own parachain for launching a t-dApp. But parachains come in handy in certain specific scenarios, because they have the capacity to execute a larger number of transactions simultaneously. Moreover, parachains have the ability to decide the transaction fees (high, low or none) at their discretion. For example, if you have some service that holds non-financial data that you need to communicate, and you expect multiple transactions simultaneously, the best solution is to run your own parachain.