Radiant gets +1/+1 for each other creature on the battlefield with flying.
Build a wide board of flying creatures so Radiant grows huge, then close with evasive beats backed by mono-white's strong protection and anthem effects. You play a tribal aggro/midrange game, flooding the air and turning Radiant into a massive vigilant threat that blocks and attacks profitably.
Vigilance plus a scaling buff makes Radiant both a strong attacker and an excellent blocker against other fliers
Fliers tribal in white has deep support: anthems, lords, and token generators
Mono-white access to cheap protection, board wipes you survive, and efficient removal
Self-sufficient commander that grows just from any fliers (including opponents') being in play
Mono-white lacks card draw and ramp, leading to topdeck struggles in long games
Commander offers no built-in card advantage or recursion
Vulnerable to spot removal and edicts that strip your evasive threats
Symmetrical flier buff can also pump opposing fliers as blockers