Actually half an inch and .5 inch are technically the same, it is the precision of the dimension that matters
The "half an inch" or ".5 inch"
is the precision of the dimension, and can mean that those two expressions of the dimension could represent quite different values.
"Half an inch" suggests a precision of +- "one quarter inch", because greater precision than one quarter inch either side of the value is
not expressed. So something expressed as "half an inch" with no further statement of precision, would be acceptable if it was greater (just) than one quarter inch, and less than three quarters of an inch. If "half an inch, plus or minus one sixteenth of an inch" is expressed, then things are different.
".5 inch" is a much more precise dimension, and suggests a precision of +- .05". Therefore, without a further statement of precision, ".5" would be interpreted as .45" to .54", which is much more precise than the .25" to .74" which is conveyed by the word expression of "half".
To take it further, .001" means a dimension from .0995" to .0014". Parts in your engine are made and maintained to greater precision than that. If you accepted a Lycoming cylinder, with a valve guide dimensioned at "half an inch", you'd be a fool!
In aviation, this does not matter much in inches, unless you're building or maintaining the plane. But, think of it in "tanks". Would you be content to have a "half tank" of fuel put in, based upon reading the fuel quantity indicator in the instrument panel? Or, would you feel more comfortable ordering .5 of 100 liters, knowing that in doing so, you must be pumped between 45 and 54 liters. Or, request .50 of 100 liters, and you'll have to be pumped between 49.5 and 50.4 liters.....