The link between tachi waza and bukiwasa can't be understood unless you learn a proper system with a clear pedagogy in it.
For instance the link between katate gedan gaeshi and ikkyo is obscure unless you practice a system, a grammar that will reveal its underlying logic with time, questionning and keys of decoding - for it is heavily crypted. Needless to say that the teaching of the different arts O sensei learnt is
integrated in a syncretic way that makes his eventual synthesis fundamentally different with the primary systems (jo or iai, jodo, Hozin, Yagyu, etc), whilst integrating many aspects of them (one interesting study is to find out
what has not been kept 
. )
Properly taught the kata (I include suburi) has many functions / benefits:
- grammar and conservatory (like a thesaurus)
- training and conditioning of the practitioner
- transmission, so that the knowledge is passed with as little loss as possible.
There are very few competent teachers around; as I sometimes say, Aikido has become a matter or archeology... Good search
