Meter was defined as 1/10M of the length from equator to the pole. It’s not exact because the size was established by people literally measuring the distance while traveling. They got really close though. Rest of the units were derived from that length such as 10cm cube of water is 1kg and so on.

Meter is pretty useful though, a large step step is around one meter so you can measure a field by walking after a bit of practice to get the step right. The height of people’s waist is around 1m also.

The point you make about counting is not really an advantage since in metric we count in meters, 10s of centimeters, centimeters, 100s of meters or kilometer which is roughly 10 minutes walking.

The difference is that when you need to do calculation or anything scientific it becomes super easy to convert between units. A cubic meter of water is 1 metric ton (1000kg), 1L of water is 1kg so you can just measure 250ml of milk (~ a cup) by adding 250g of water to a bowl. A tablespoon by adding 15g and a teaspoon by adding 5g.

It makes cooking and baking a lot easier since you don’t need to bring out a whole range of measurement equipment because you can often just use a scale instead.

Then we have Fahrenheit temp scale. Goes from 0 very cold and 100 very hot. Both are intuitive after a while but metric make it very easy to reason about larger ranges and how close to freezing something is outside.

Bottom line is that both are very arbitrary but any time you go outside some regular guesstimates of “this is x long” metric becomes easier.