Yes, the purpose of S-300/400 is quite obvious/trivial, as per
https://en.wikipedia.org/wiki/S-300_(missile) :
"The S-300 system ....., designed for the air defence of large industrial and administrative facilities, military bases, and control of airspace against enemy strike aircraft".
However, it was announced today in Russia that Dailymail is wrong, as usual: there are no S-400 batteries in Syria as of now, and the foreign guests who were invited to the Russian airbase in Syria and made these pictures just did not recognise correctly what they shot on cameras:
Russia Denies Rumors of S-400 Air Defense Systems' Deployment in Syria
Anyway, the truth will come out soon as S-400 are not pocket guns and can't be hidden for a long time if they are there.
IMHO, it would more than enough to have modern Buks and Pantzyrs as air defense means in Syria if you are not supposed to work on "stealth" machines, which are only in the US possession (F-22, -35 and B-2).