Physical significance of pumping length in pumping lemma
First, in the Pumping Lemma as I know it, there is no such thing as the pumping length for a language. "For every regular language $L$ there exits a constant $p$ such that for any string  $w\in L$ with $|w|\ge p$ ...". In fact, if $L$ satisfies pumping length $p$ then it will also satisfy any pumping length larger than $p$. (This is not completely trivial, the requirements in the dots ... above mention $p$ again.)

So we can call the smallest length $p$ for which the pumping property holds "the" pumping length. This is indeed a property of the language. If I am right the pumping length for $L = a(aa)^*+b(bb)^*$ is three. Strings of length 3 and above can be "pumped".

As you observe yourself, every regular language can be pumped (well, that is what the Lemma states). And the proof may use any automaton for the language. Whenever a string is at least as long as the number of states in the automaton, its computation must repeat a state, and we can pump. We can use the smallest automaton for the language, to minimize the pumping length. That smallest deterministic automaton is in fact well-defined. Unfortunately this in general still does not fix the (minimal) pumping length. An automaton for $L$ above seems to use at least five states (and actually 6 if it must be deterministic).

But still the property seems tied to the regular language via a certain "minimal" automaton, and the length of strings that ensure repetition of states.

But hey, even non-regular languages might in fact be pumpable: the Pumping Lemma (or better Pumping Property) is not a characterization of the regular languages. See for instance "Languages that satisfy the pumping lemma but aren't regular?" where it is observed that languages of the form $c L\cup \{ c^k \mid k\neq 1 \}\cdot\{a,b\}^∗$ for any $L\subseteq \{a,b\}^*$ have the Pumping Property (for pumping length one or two, I believe).
I do not know a "physical" significance in the nonregular case.