As it is with every vendor, new major releases (18.1, 18.2, 18.3, ...) bring new features and bugfixes. Maintenance releases (R2, R3) will bring bugfixes and sometimes new features, and Service releases (e.g. R2-S1, R2-S2) will bring bugfixes only. And all of them can include security fixes.
Please note that JUNOS releases have End Of Life dates, these are important if you have service contracts and need to open a JTAC case. You can find the EOL dates here:
Just FYI, but when looking for Junos SW versions, I would suggest the 1st step always would be to change OS to Junos SR from default of Junos. I tried to get this changed, no luck -:( Then select code stream. S/SR releases are 90+% the normal for most cases, especially for EX and QFX products. I think EX/QFX are at 100%.