The simple answer is, for getting punishment of the committed sins.
But then the question becomes, as you have raised, why someone would experience the results in hell when he has to also experience the results of his bad actions on earth itself? This is a nice question. It is known as the problem of double retribution. To understand the reason of why or why not, we have to first understand what causes a sin.
For smooth and proper operation of life here and in the hereafter the rishis formulated various laws. Through their power gained through austerity they could perceive past, present and the future, so they knew what activities lead to what conclusions. They were aware of which activities break the order and what could be done to mend the order. Through various Puranas and scriptures they they gave out these laws for the benefit of the humans. So whenever any set law of the sages (varnashrama dharama etc.)is violated, it creates a sin. And for breaking the order or the rule one goes to Naraka for punishment:
varṇāśramaviruddhaṃ ca karma kurvanti ye narāḥ
karmaṇā manasā bācā nirayeṣu patantite [VP - 2.6.32]
He who through his mind, speech and action does work that is against his varnashrama dharma (duites as per his caste) falls into hell.
However, sometimes a law may get violated unintentionally or out of necessity. So as a counter action, the sages have also stated the actions that should be done in such cases. The set of actions done for the expiation of sins is known as prayaschit.
Sages prescribe a penance for unintentionally committed sins. Some
declare, on the evidence of the revealed texts, (that it may be
performed) even for an intentional (offence). [Manu. Smrt. - 11.45]
Apart from other bad actions, some grave acts like (murder of Brahmin, mating with someone forbidden by the scriptures, drinking alchol etc.) when done consciously has no other expiation than death. So here on earth itself one can free himself of sins by proper expiation. If however, on committing the sins, one doesn't expiate for them on earth, then he has to experience the corresponding punishment in hell as mentioned in various scriptures:
pāpakṛdhyāti narakaṃ prāyaścittaparāṅmukha: [VP - 2.6.38]
He who commiting a sin who doesn't want to expiate, goes to naraka (hell)
So the result of certain grave actions are not experienced on earth but in other planes of existence like hell and heaven. It's because they can not yield equivalent result here on earth. For example, when one performs 100 asvamedha yajnas then the resulting merit becomes so big that he can't enjoy it here on earth. So he has to become the king of heaven. Similarly certain bad actions and severe actions like murder of people are so grave that they can't have equivalent effect here. So for getting equivalent punishment they have to go to hell. Only the result of minor good and bad actions are experienced here.
Mahabharat Apaddharmanusasana Parva, Chapter 165
For an overall idea refer Garuda Puran, Agni Puran, etc.