in the end of the adjectives, numerals, pronouns of genitive case of male and neuter -ого/-его pronounced as [в]: большого дома (озера) – большо́[ва], синего флага (моря) – си́не[ва]. The same rule we use for the words сегодня – се[в]о́дня, итого – ито[в]о look through a website www.evartist.narod.ru/text1/73.htm

we have this rule in order to make the words easier and softer to pronounce. It sounds better) the same as we have in English the article which we pronounce in a different way here- the book and here- the others.

As someone also wrestling with this, I guess it comes down to common use. Very frequent words such as сегодня and the -ого ending would be very tiresome with a hard г. :)

Many centuries ago, South Eastern Slavs (now Byelorussians and Ukraineans) pronounced the letter "г" as a voiced velar fricative /γ/. North Eastern Slavs (now Russians) could not pronounce this sound. In most words they began to pronounce the letter "г" as a voiced velar plosive /g/. However, in some words they began to pronounce he letter "г" as a voiced labiodental fricative /v/.