Why unintended capacitances are an issue for the design of chips?
Higher capacitance inside the IC has several effects, among them:

1) Signal degradation. A sharp pulse going thru an RC filter will go out "less sharp". Namely, it if had a fast rise-time at the input of the RC, at its output the rise time will be slower. If it is slower, it may grow not fast enough so that the receiving end recognizes a change from '0' to '1' or viceversa in time (*)(a conductor between a transmitter and a receiver can be modeled as a series resistance and a parallel capacitor in the termination, thus the R-C).

2) Crosstalk. Capacitance between conductors is one of the causes of crosstalk. Crosstalk means that the data changing in one conductor affects the data on a neighbor conductor. This can corrupt the data or at least worsen its signal to noise ratio.

(*) Digital circuits change all the time with fast clocks. If a signal gets slowed down because capacitance in the conductor, it may not change fast enough for the receiver and the next clock pulse may sample a wrong value of the 'slowed down' signal.