This is called "concatenation".
I would also note that many people use the Person Notes or Comments fields if they exist in their system already.
I would also be careful that while the Trigger will do what is expected here, most people will use this as the flow steps from a Form Fill rather than looking at this field change directly.
Sanford Whiteman wrote a great blog post awhile back that helps make these type of fields more readable by adding a line break token:
Requires a bit of digging into firefox inspector but once setup it is very useful for cases like this.
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This line-break hack has been so beneficial in being able to store readable values in Marketo. In addition to this, there's another technique (by Sanford, of course) to render the hidden line-break into a <br> tag so that this same information can be made readable in our real-time alerts. I spoke to this in great detail during my Summit presentation:
Another note is that many people would refer to these temporary or temp fields as proxy fields that are intended to hold short-term data and either be cleared out as part of your flow steps or simply overwritten and are never taken into account for reporting purposes.
I have utilized proxy fields for string, datetime, and otherwise in the past and have usually set them up as a CYA for one-off needs.