Forex

Hello guys
I'm very confused of why we sell/buy currencies using forex ...
I understand that countries econmoies detrmine the value of their currencies , so , when I buy it and the country economy increase in value my currency will increase in value ...
is this identical to the case when I buy the currency manually from the nearest bank in my city offline ??
if this is the case , what Tokyo , London ..... sessions and why should I wait for them and even weekend close..
is not this like the situation ( offline buying ) but this is online ?@

Please Help me i'm v confused !@@!