I believe you're looking for the word "use".
I know that seems overly simple, but I've been in construction for almost 40 years and the designation of a buildings purpose, residential, commercial, retail, office etc. is called its "use"
I assume you are referring to structures that are left over and serve no apparent purpose. These are called vestigial structures.
They are purposeless structures
homologous structures
Similar structures that have evolved independently are called homologous structures. An example would be the wings of a bird and the wings of an insect.
Homologous structures.
chromosomes
so we can see what you write
homologus structures
Similar structures that evolved independently are called analogous structures, or analogies.
Gases enter plants through the Stomata, microscopic mouth like structures on the bottoms of leaves, for the purpose of inhaling CO2 for Photorespiration.
The structures are called chromosomes.
The threadlike structures are called rhizoids.