Why should text files end with a newline?

Reasons:

UNIX standard
If the last line in a file doesn't end with a newline then addition of next line affects two lines instead of one. This also pollutes diff on multiple files, so reader may wonder what has changed in a line whereas no significant change has occured.

Multiple newlines at the file end are also redundant as well as spaces at the end of line.

How to fix properly and once

Ensure your IDE/editor has the relevant setting set:

IntelliJ: navigate to Settings > Editor > General and
- check Ensure line feed at file end on Save
- set Strip trailing spaces on Save to Modified Lines
VSC: navigate to Preferences > Settings and
- search for "newline" and check the following
  - Files: Insert Final Newline
  - Files: Trim Final Newlines
- search for "whitespace" and check Trim Trailing Whitespace
Sublime Text: navigate to Preferences > Settings > Default find and set the following to true:
- trim_trailing_white_space_on_save
- ensure_newline_at_eof_on_save
- ... or use Single Trailing Newline package to ensure one and only one newline at the end of file

Other IDEs may have a setting similar to any of the above.