1. On hibernate system save all data from ram to swap.
2. When ram is full, swap is used as a secondary ram
Recently released ubuntu uses caching in ram. It's means save recently closed application data in ram, so that when you will use the application again it can load it quickly. Cache increase performance. For that ram becomes full (2GB or less). When ram fill with cache and system needs more ram it backup old cache in swap. If you don't have a swap then your system behave slow when ram is full.
What happen if you have already installed your system without swap partition? Fortunately ubuntu has option to create a swap as file.
Step 1: Allocate a file sudo fallocate -l 2048m /mnt/swap_file.swap
Step 2: Change permission sudo chmod 600 /mnt/swap_file.swap
Step 3: Format the file for swapping device sudo mkswap /mnt/swap_file.swap
Step 4: Add swap sudo swapon /mnt/swap_file.swap
Step 5: Open fstab sudo gedit /etc/fstab
Step 6: Add entry in fstab /mnt/swap_file.swap none swap sw 0 0
source : here