Arcade shines brightest when it is used to overcome schema issues out of your control. Using simple tests, one is able to effectively clean/screen data for datasets that are not part of their jurisdiction/responsibility. As part of that strategy, a tool like isNumber() could be used to help screen data.
A specific use-case would be setting angles for cartography, if the only field available is a string, then you can write a test like:
WHEN(
ISEMPTY(thefield) == true, 0, //to test for nulls
ISNUMBER(thefield) == true, NUMBER(thefield) , \\to cast the string field
0 //default for records that consist of only text
)
The current alternative is below and can be unintuitive for individuals without javascript experience:
WHEN(
ISEMPTY(thefield) == true, 0, //to test for nulls
ISNAN(NUMBER(thefield)) == false, NUMBER(thefield),
0 \\default for records that consist of only text
)
ISNUMBER( value, pattern? )
P.S. Shouldn't ArcGIS Arcade have its own Ideas Community?