More requests to your site means your web server creates more threads to handle those requests. Since each thread will require it's own memory allocation to serve the requested content your resource needs will increase as the volume of traffic increases.
Droplets do not have swap space by default. Swap allows the system to use some of your disk to "swap" out some items in physical memory so that you do not run out.
Without a swap file if you utilize all of the RAM on your system you will likely see processes begin to crash as they are unable to allocate memory.
The RAM allocation is not just a subscription or plan limitation. Your droplet is only assigned what you pay for and cannot exceed that unless you upgrade to a larger plan.