Indicators 'attached' to a chart & including & embeded in the EA - vs - making an 'external' call to them?

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Howdy All,
From what I have read, one doesn't HAVE TO include indicators within the EA nor even attached to a chart in order to utilize them, but can make 'external calls' to them when needed.
I'm wondering about the 'Pro's & Con's' and 'trade offs' of these two different methodologies?
Thanks to any and all for your thoughts, knowledge and assistance with this!