It is indeed a desirable trait to keep oneself spiritually holy at all times. One should study, pray, and do Mitzvot to the best of their ability. This is an admirable characteristic. Aharon the Kohen Gadol was probably one of the holiest men in Jewish history.
Yet the Torah in today’s portion restricts him in a sense from pursuing this conduct exclusively. Hashem warns Aharon: VE’AL YAVO VECHOL ET EL HAKODESH, “…he shall not come at all times into the Sanctuary…”. (Lev. 16,2) In a homiletical sense the Torah is saying that Aharon should not be constantly engaged in spiritual matters.
A spiritual leader must be ready to step down to the level of the average person and get to understand their needs and their hopes and aspirations. Not everyone can reach the heights of a spiritual leader and it is thus incumbent upon the leader to step down and see how the other person lives and what their spiritual needs are.
Hashem tells Aharon the Kohen Gadol that you must not always be engulfed in spirituality but must be able to see things in this mundane world and deal with them appropriately.