Snapchat may be looking to add payments for stuff through PayPal as Snap was discovered to have installed the PayPal SDK which seems to signify that it has plans to add in some sort of e-commerce or payment solution into the app. Snapchat already has a person to person payment system in place called Snapcash, so this could be something completely different and is possibly geared towards users being able to pay for goods or services of some kind. Of course there are other possibilities as well and at the moment Snap may have no plans at all for using the SDK.
If Snap is planning to integrate PayPal with the app it's speculated that it may be using PayPal as a way for users to purchase the Snapchat Spectacles. This could very well be a likely scenario as Snap already now sells the Spectacles online in the U.S. and is expanding the popup Spectacle machines in other regions outside the U.S., so perhaps it's preparing for a global rollout by letting users anywhere the app is available buy Spectacles through the app.
Another possible scenario could be that Snap is planning to use PayPal as a method for a new ad platform. If this is the case, PayPal might be a method of payment that brands will accept if they sell items through Snapchat to its library of users. PayPal is a widely used payment method for tons of online and in-app purchases, so it would make sense if Snap decided to integrate this sort of feature, as it would allow brands to use Snapchat as a way to reach users and sell products to them directly. Snap already has an ad platform set up for advertisers to reach new and existing customers, but integrating PayPal and allowing brands to sell to users directly through the app would potentially be more efficient for brands as they could be advertising and selling in one location. One last possible option is that Snap is replacing Square as the technology behind its Snapcash system. At the moment there's no telling what Snap's plans are for adding the PayPal SDK into Snapchat, and it's possible that it has no plans to use it for anything at this point, but at the moment it's important to remember that there is no confirmation on what Snap may be doing with the SDK right now.