Left : 3 GB RAM for Samsung Note 3, Middle : 4GB RAM for Samsung Galaxy S7, Right : 1GB RAM for Motorola E 3rd Gen
RAM is one of the underrated features of Android phones when it comes to buyers... Most focuses on how many cores it has, how big the screen is, OR how big the battery is. That they then to forget or ignore the important value of RAM.
RAM is the fast, temporary storage the phone loads the system files AND applications when the phone starts up. The more RAM, the better. More RAM means that more apps, can be loaded on it, without needing to reload the app from the internal storage (aka. ROM). When RAM becomes used up, it suspends apps that are not being used. On mid level phones (1GB+ RAM), it wont affect performance much. But on phones that has 1GB RAM and below AND has FB, the phone will be usually busy on suspending / unsuspending apps in the background, which affects performance and battery life.
There are two types of apps in Android, apps that loads up on RAM when launched, then exits when closed or terminated (ex. MX Player, VLC, Google Maps, etc) and apps that are persistent, keeps loading itself on RAM even if closed OR terminated (except when Forced Closed, then its gone until relaunched), most of messaging apps (FB Messenger, Hangouts, Viber, etc) and Social Networking apps (Google+, Facebook, Twitter) are persistent type apps, which could be a good thing (if real time communication is important for you) or a bad thing (will slow down your phone drastically).
Case in point : Facebook and Facebook Messenger are known RAM hogs, several articles (Reddit LINK, Google Search LINK) claims that uninstalling FB or FB Messenger will improve performance, IF you have a high end phone (S7, G5, etc), it wont matter much. BUT if you have a phone that has 1GB of RAM or less, you will see a massive performance increase!
So what if your a person that needs FB in their life but cant stand a slow phone, and cant buy a new phone either? Well, the Facebook team heard the cries of their users and created Facebook Lite. Its the literal light version of FB. From the average 30-120mb++ usage to the 9.8mb usage of FB Lite. For the people mentioned above, its a god send.
Always remember, when buying Android phones, always consider RAM. The bigger, the better. 2GB is now usually the minimum to have a decent working phone, anything lower will cause a headache.