What does the mandatory constraint type in biblatex do?

To have these constraints checked you need to pass the --validate_datamodel option to biber. On your example running

biber --validate_datamodel file

I get

[...]
WARN - Datamodel: Entry 'myarticle' (file.bib): Missing mandatory field - one of 'date, year' must be defined
WARN - Datamodel: Entry 'myarticle' (file.bib): Missing mandatory field 'author'
WARN - Datamodel: Entry 'myarticle' (file.bib): Missing mandatory field 'journaltitle'
[...]
INFO - WARNINGS: 3

If you wish, you can set this option in a biber.conf file consisting of

<?xml version="1.0" encoding="UTF-8"?>
<config>
  <validate_datamodel>1</validate_datamodel>
</config>

instead of writing the command line option to biber each time.