It allows for some spill-over when you aren't at a full 1bil/train. E.g if it costs 800 million/train but cap is 2.4 billion you can get 3 trains in. Otherwise you have 400 million left over that you either leave and shorten gap till you have to stop again because we all love stopping mid run and then having to run back to the zone every 10 minutes (and Im a porter saying this) or you convert it and start again from scratch.
There is also something to be said for sense of accomplishment. "Ohh look I just ran all that and gained 10 health, YAY, only 99 more times to go" vs "All that and I trained 50 health, only 20 more times to go". Perception goes a long way towards keeping people from giving up as too tedious.
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