The RPi2 and 3 have 1GB of RAM, which is fairly limited even when compared to a smartphone. The Linux kernel's memory management system is virtual-based, was designed with swap in mind, and not having available swap hobbles the system by using memory less efficiently(Technically, with a backing store, as the Linux kernel doesn't swap, but pages memory. However, common vernacular is to use "swap," so I will also). The kernel is loaded into memory, and AFAIK, never swapped. The left over memory is available for user-space apps.
Apps often take up memory for initialization, and never use those pages again. Instead of wasting the memory, the Linux memory management system is designed to move those pages to swap to maximize the available physical memory. As is self-evident, in a system with a small amount of physical RAM such as the Pi, this is even more important.
Yes, having a swap partition (or file) will shorten the life span of the microSD card; every write shorten's the lifespan of a flash device. IMHO, microSD cards are so cheap that this should only be of concern to the most cash-strapped of individuals.