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"
They are purposeless structures
I assume you are referring to structures that are left over and serve no apparent purpose. These are called vestigial structures.
so we can see what you write
NUCLEUS
Homologous structures are similar evolutionary structures that evolved in different organisms that were used for the same purpose long ago but are no longer in use
pulpwood is mainly used for building houses other similar structures
cilia are little "hair-like" structures
Organizations need structures for purpose of administration and management. The consequences of not having well-set structures in organizations are quite severe and include mismanagements of funds, failure to achieve set goals and objectives and much more.
The purpose of radical neck dissection is to remove lymph nodes and other structures in the head and neck that are likely or proven to be malignant.
The skeletal structures are made of an aluminum or steel alloy and its main purpose is to provide support to any new building during construction.
Plants need their leaves to gather sunlight for the purpose of photosynthesis.
The pillars of Aksum were built. Their purpose was to celebrate great victories and achievements. The writing on them stated what great things had been done.