Jaques and Touchstone have similar purposes in one regard in As You Like It, but have very different purposes in another regard. First, they have similar purposes in that, though of different backgrounds and stations in life, both serve as diversion and amusement for the principle characters attached to them. Jaques, though a lord in exile with Duke Senior, diverts the Duke with his melancholy and with his wordy witticisms--the wit of some of which he may be unaware.
On the other hand, though Touchstone similarly provides amusement and the diversion from word play for the royals he serves (first at court and still in the Forest of Arden), he has always had the lowly station the court jester in motley (multi-colored clothes) as the Shakespearean Fool. The major difference between the Shakespearean Fool and Clown, aside from being urban and rural, respectively, is that the Fool intends to make word play and be witty whereas the Clown does so unintentionally by accident rather than by choice. By this part of the definition, Jaques might reasonably considered to be fulfilling the office of accidental country Clown for Duke Senior's group of pastoral exiles.
The major difference in their purposes is that Touchstone, whether he wants to or not, whether by design or by accident, reflects back the worth and truth, if there be any, of the person conversing with him, whereas Jaques provides an alternate point of view about the pastoral life of the exiles in Arden. First, a touchstone (a geologic stone) is so named because when gold or silver is rubbed on it a distinctive mark is left behind that identifies the true nature of the metal. This was quite important in eras where metals of inferior origins were passed off as gold and silver. Similarly, Touchstone's conversations with other characters display the worth of their points of view and their beliefs. A good example is Touchstone's conversation with the old shepherd, Corin, in which the truth and sincere simplicity of Corin's believes is made evident since Touchstone can't offer an opinion, witty or otherwise, that can stand up to Corin's statements.
Jaques' melancholy spirit and gloomy conversation, on the other hand, stand as alternatives to the prevalent points of view in the play. All the other characters, except perhaps Touchstone, are all delighted with the pastoral life and the life of simplicity in Arden, which includes the deposed Duke Senior and his exiled lords. However, as a good example of his purpose, Jaques disagrees with the exiles' belief that the forest creatures, such as the deer, are there to provide food for them and that the death of a deer is a sad but necessary event. Jaques presses the point that the exiles are exerting the same rule of order that drove them into exile, the law of "right by power." In the end, Jaques yields to his view of life and chooses to explore exiling himself into a truly pastoral monastic life.