What Plain Text is
Plain text here means columns lined up with spaces, the way a command line tool prints a table.
Terminal output, log files, fixed-width exports from older systems.
What PNG is
A lossless image format. Text and thin lines stay exactly as drawn, which is what a table is made of.
Slides, chat messages, issue trackers, and anywhere a table has to travel as a picture rather than as data.
What changes when you convert Plain Text to PNG
Two shapes are handled. If the same character positions are blank on every line, those gutters are treated as column boundaries, so values containing single spaces stay intact. Otherwise the lines are split on runs of whitespace.
The table is drawn from the data onto a canvas rather than captured from the page, so the whole thing comes out however long it is and however much of it is on screen. Column widths are measured from the content, alternating rows are shaded, and the scale is adjustable so a table for a slide or a print layout can be drawn at two or three times the size.
What carries over from Plain Text to PNG
plain text records 2 things about a table that PNG has no way to hold.
Nothing in PNG records that the first row is a header. It is drawn at the top and looks like one, which is all that is left of the distinction.
PNG is the end of the line for this data. Reading the table back out of it is a separate and much harder problem than writing it, so keep the plain text if you will need the values again.