Just as Sean said, unused ram is useless ram.
It's all about access speed, the more data you store in ram the more content is quickly available.
Flash memory has an average transfer rate of 5 to 7mb/s while ram has an average transfer rate of several gb/s.
The difference in speed here is very noticeable and the main reason why recent android versions (or Linux in general) caches so much data in ram.
Long story short, the more ram is in use, the faster your phone gets.
400mb is basically the safety threshold that it keeps unoccupied just to be sure it has some left in case of an emergency (you trying to open content that isn't already cached).
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