There are two categories of martial arts: Internal and External. However categorizing different styles into them is not very easy as there are many overlaps and almost all the genuine styles have both internal and external aspects to various degrees but anyway Taijiquan(tai chi) along with xinyi, bagua and a few other styles are traditionally called the internal martial arts. Theses styles were not really created or developed for a spiritual purpose and their sole goal like any other martial art was to train the body to be more effective in fights. However in order to be more effective in using all the power of the body the creators of these arts started incorporating the (mainly)Daoist philosophy and their qigong meditation in their martial skills. So in some ways meditations that were created for becoming immortals (or something like that
) were preserved and passed on in the form of martial arts more like a side effect but after guns and western weapons changed the way of fighting, since there was not much need for martial arts anymore, the health or spiritual aspect gradually got popular especially in Yang style of Taijiquan.
As for your question, I think it really depends on how you learn and practice Taijiquan. All kinds of physical training can help in moving and saving the qi in your body and if taijiquan is just practiced as a series of dancing moves then it will give you the same benefit that you can get by going to an aerobic class or anything like that, the only difference is that it is more suitable for the elderly because of being relatively slow. However in order to get the full benefit you should find a good teacher from a legitimate lineage. The training for that kind of taijiquan is difficult and needs spending a lot of time on both the meditation/qigong aspect and hard physical training so that your three external harmonies(the particular body mechanics of taiji, xinyi and bagua) and your three internal harmonies can get connected.
Edit: BTW I know that in English it is usually called tai chi but to avoid any confusion I just want to mention that in Mandarin 太極 is pronounced "tai ji" and as you see it's different from 氣 qi(pronouced chi in English) energy.

As for your question, I think it really depends on how you learn and practice Taijiquan. All kinds of physical training can help in moving and saving the qi in your body and if taijiquan is just practiced as a series of dancing moves then it will give you the same benefit that you can get by going to an aerobic class or anything like that, the only difference is that it is more suitable for the elderly because of being relatively slow. However in order to get the full benefit you should find a good teacher from a legitimate lineage. The training for that kind of taijiquan is difficult and needs spending a lot of time on both the meditation/qigong aspect and hard physical training so that your three external harmonies(the particular body mechanics of taiji, xinyi and bagua) and your three internal harmonies can get connected.
Edit: BTW I know that in English it is usually called tai chi but to avoid any confusion I just want to mention that in Mandarin 太極 is pronounced "tai ji" and as you see it's different from 氣 qi(pronouced chi in English) energy.