If you catch a Pokemon from a raid battle you will get exactly the same Pokemon, the same natures, IVs, gender, moves, etc.
However they will not have things that are exclusive to Raid Battles such as being Dynamaxed for the entire battles or being able to have shields. That is not the Pokemon itself becoming weaker, it is exactly the same data-wise. The only difference is the one in the Raid Battle is the Raid Battle opponent, while the one you catch is your own Pokemon. Although there are two exceptions: If you catch a Raid Pokemon higher than the amount of badges will allow you to catch, it will scale back to the max level you are allowed. Second is in the Dynamax Adventures from the DLC where the game has a chance to force Pokemon to be shiny.
There are plenty of reasons to do Raid Battles regardless. Simplest reason is that if you want a Pokemon in that battle, you now have an option to catch it. Some Pokemon in Raid Battles may have their hidden ability and perfect IVs. Gigantamax Pokemon can only be obtained through Raid Battles without DLC, and even with DLC you need to complete Raid Battles to spawn Max Mushrooms. Even if you do not catch the wild Pokemon, you will still be awarded with items including EXP Candy and TRs.
Source: https://bulbapedia.bulbagarden.net/wiki/Max_Raid_Battle
