The setting 'Manual Update' stems from a very long time back, for as long as I can recall and having used Avast for 15 years, when dialup was very common. Avast was also much smaller, less functionality, etc. and the default settings back then were different. The VPS Updates were set at 'Ask when an update is available' Notify, commonly there were only 2 a day, but often there were more.
Even the Program Updates if memory serves me well had the same setting. There were no streaming updates and there were no Emergency Updates. The Emergency Updates are completely separate from the Program and VPS Updates, they don't rely on your settings and it is a scheduled task. This makes a connection to check for the presence of an emergency update, if it is present it will be downloaded and installed.
You can monitor the defs sub folder and see if you are getting any streaming updates (image1), I rather doubt that you are. Inside one of these stream sub folders you can see the frequency of these very small streaming updates (image2). These combined with your last VPS Update keeps the virus database fully up to date and I believe may even speed the actual VPS Update download.
VPS Updates can also deliver engine updates and or bug fixes to save having to do a full program update.
Much has changed in the world of malware and AVs have also changed to meet this changing threat.
Obviously it is your system and your choice.