The Launchpad board includes some debugging hardware that allows you to debug the microcontroller using only a USB cable. This makes it a very affordable eval board for TI's customers because they don't have to buy a special debug cable.
When a developer designs a custom board for the MSP430, they don't typically include the debugging hardware that is included on the Launchpad eval board. This makes their custom board more affordable in large quantities because they don't have to include those debugging related parts on every board. But then in order to debug the custom board they typically buy one of the special debug cables. One debug cable can be reused to debug all the custom boards.
If you're making something that you plan to make only one or a few units of then it's most affordable to buy the Launchpad and design a booster for any specialized hardware that isn't already included on the Launchpad. (A Launchpad booster is analogous to an Arduino shield.)
If you're designing a board that you plan to make many units of then you'll eventually want to invest in one of the specialized debug cables. But even in this case it could be very helpful to start off with a Launchpad and use it for some early software development while you're waiting for your custom boards to become available.
It might be possible to use the Launchpad to debug your custom board (in place of the special debug cable). I'm not familiar with that but you can probably search for information.