Why should we use arrows.meta instead of arrows?

Question

From the TikZ-PGF Manual, Section 16.1:

Remark: The libraries arrows and arrows.spaced are deprecated. Use arrows.meta instead/additionally, which allows you to do all that the old libraries offered, plus much more. However, the old libraries still work and you can even mix old and new arrow tips (only, the old arrow tips cannot be configured in the ways described in the rest of this section; ... omissis ... )

We have posts which explain:

• why it's better to use \tikzset instead of \tikzstyle
• why it's better = of instead of of =
• how to migrate from arrows to arrows.meta,

but we have no post which tells us why we should use arrows.meta instead of arrows.

Since the latter is one of the beginners' "bad" common practices, I think that a little list of the "much more" offered by the former may be useful.

Answer 1

As far as I am aware of, there is no concrete reason (except a few corner cases about misalignment and scaling) that requires the word "should".

However, as many times asked and finally included in TikZ, in Is it possible to change the size of an arrowhead in TikZ/PGF? Luigi went a great distance and implemented a new parametrization of the arrowheads together with many new arrow templates. Then, I don't have the whole story how it went, T.Tantau took it in either the code itself or rewritten in the official release. In the feature request ticket that Luigi proposed, T.Tantau writes

On 2013-08-29:

Many thanks. I have been thinking about how to integrate this. It seems like a good idea that a general "arrow tip size" should be an always present arrow option, which would simplify most of the code. When I have the time, I will address arrow tips in general and then integrate this.

and then on 2013-09-18:

The whole arrow tip management has been completely rewritten (but not yet documented). I am busy adapting the old definitions; the most important arrow tips are already adapted and one can now configure the size and shapes of arrow tip "to death" and on-the-fly. Many thanks for the code.

Old arrows library does not offer any parameterization of the arrowhead but instead derives most of the arrow parameters from the linewidth of the path it is sitting on.

In the new library almost every aspect is customizable.