In your own words, explain why seconds are derived from an actual value and not an arbitrary one.

We have that The second was derive from a number that is

of a solar day(24hours)

it is important to note that the seconds was discovered or established before  clocks

Now, seconds are derived from an actual value and not an arbitrary one.

The second was derive from a number that is of a solar day(24hours)

The second was designed to make the its value as close as possible to this number in the modern day seconds

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Second is a unit for measuring time. As the scientist usually wants precision, is needed that the units are well defined and that the unit is exactly the same for everyone.

So, if for example, (and for some weird reason) you do not have access to a precise clock to measure one second, you can use the actual value (by definition) of the second to find the exact length of a second.

So, resuming is used for consistency.