Internet in some regions of Indonesia can be inconsistent. A batch download allows you to save the files to a hard drive, laptop, or tablet. You can watch the “Weirdmageddon” finale on a plane or in a remote area without buffering.
Regarding the "Sub Indo Batch," I assume you're referring to a batch of episodes with Indonesian subtitles (Sub Indo). There are several websites and online platforms that offer Gravity Falls episodes with Indonesian subtitles, but I must remind you that I couldn't find any official sources that provide the show with Sub Indo. Gravity Falls Sub Indo Batch
Here's a clean, organized answer for you: Internet in some regions of Indonesia can be inconsistent