grammar

[puzzle]

I. All magnets behave the same, be they large or small.
II. She'll be sixteen years old, come May.

I can't understand why use inversion structure in these two cases? Please.

[Niall]

You can interpret them as follows:
I. We need to add the conjunction whether:All magnets behave the same, whether they are large or small.
II. Adding when: She'll be sixteen years old, when May comes.

[RonBee]

You can say it however you like.

Whether they are small or large, all magnets behave the same.
In May, she will be sixteen years old.

[apex2000]

I. All magnets behave the same, be they large or small.
II. She'll be sixteen years old, come May.

I can't understand why use inversion structure in these two cases? Please.

It is simply a manner of speech, colloquial. You do not have to use inversion.