Here are the most common reasons:
Streaming video: YouTube, Netflix, and video apps use 300-500MB per hour—this is the biggest culprit
Auto-playing videos: Social media (TikTok, Instagram, Facebook) auto-play videos by default. Turn this off in app settings
Background app refresh: Apps update in the background. Go to Settings and disable background refresh for apps you don't need
Cloud sync: Photos, email, and backups can silently consume data. Disable auto-sync or use WiFi only
OS updates: System updates can use significant data. Do these on WiFi instead
Streaming music: Spotify and Apple Music use 5-10MB per song—high quality uses more
Quick fix: Switch video quality to "Low" in YouTube/Netflix, disable auto-play on social apps, and only sync important apps.