You do in case something happens when you're trying to crack the case open.
Telling the manufacturer, "Well, I didn't think the CPU you placed in it was up to snuff, so I tried replacing it myself, but now it doesn't work" is going to get a very different response from them than telling them, "Well, I sent the laptop to you so that your tech people could upgrade the original CPU, but your tech people apparently screwed something up because it's no longer working", especially if it's still under warranty: the latter gets you a, "No problem, we'll replace it for you since it's still under warranty & our guys screwed it up", the former gets you a "Sorry, but you voided the warranty by taking it apart yourself, so you'll have to pay for a new laptop yourself".
And I've never heard of anyone refer to taking apart a laptop, especially to upgrade internal hardware, as being "easy"...least of all people who actually know what they're doing. If it was, the local Micro Center's "Build Your Own" section would have more than just hard drives & RAM for their available laptop upgrades.