The minds and hearts of the people who offered up sacrifices in the Temple were spiritually elevated, even if the people themselves were not consciously aware of it. This is because our Divine soul is always aware of the true state of our relationship with G-d.
Thus, the Torah does not need to explicitly state that the physical offering must be accompanied by a spiritual, inner offering - because this is always the case, from the soul's perspective. Instead, the Torah begins its discussion of offerings with voluntary offerings, since these clearly involve the offerer's mind and heart. This indicates that all offerings, even obligatory ones, are essentially voluntary offerings, stemming from the soul's innate desire to come close to G-d.
The same is true of our prayers, which correspond to the daily sacrifices. Our minds and hearts are always involved in our prayers, even when we do not consciously sense this involvement. Thus, we need never feel that when our minds or hearts "wander" during our prayers that nothing was accomplished.
This awareness can inspire us firstly to cultivate the habit of praying regularly (no matter how well we succeed in concentrating), and secondly to invest more effort in involving our conscious minds and hearts when we pray.
From Kehot's
Daily Wisdom #2