The passage to which you refer, "Mark 16:17-18", must be understood in the context of God's plan.

God has never wanted his people to live in a state of confusion, so during transitional times he took steps to ensure that people had the means to know who to listen to.

For example, when Moses brought forth a new law, he was given the ability to perform miracles so that the people could know that he spoke in behalf of God.

Similarly, when Jesus entrusted the disciples to spread the gospel all over the world he knew there would be rampant skepticism and downright opposition.

So he gave those original twelve apostles special powers. They could perform all kinds of miracles, and they could confer a limited miraculous gift on someone by "laying on" of their hands. This way, during the transitional period between the established ("Old Testament") law and the completion of the bible which would establish the new one, people could know who to listen to because they were able to confirm their message through miracles.

Evidently, only the apostles had the ability to lay their hands on others to confer miraculous power. And the individual receiving such a gift would receive only one particular type of power. As an example, they might be able to speak in tongues but not necessarily interpret tongues. Or they might be able to heal diseases. Or they might receive the gift of prophecy.

In I Corinthians chapters 12 and 13 the writer is discussing these various gifts and their usage in the congregation. It appeared that some of the people who had "flashier" (and more useful) gifts (such as the ability to heal diseases or perform other miracles) would get lots of attention while others who had only gotten, say, the ability to speak in tongues wouldn't have much to do. After all, if everyone in the assembly shared the same language, what would be the point of someone addressing them in a different language?

So the writer made a simple rule: If there was nobody there who could understand the tongue-speaker then the tongue-speaker was to refrain from using his gift.

But the writer also told them of a time in the not-too-distant-future when all those gifts would disappear. "For now we know in part, we prophecy in part. But when that which is complete is come, that which is in part shall be done away with".

These various miracles were (once again) God's way of making sure that the people knew who to listen to and who to be skeptical of during the transitional time when everything pertaining to the new law had been established.

But once it was all established the miracles would go away. The completion of the bible made it no longer necessary to have these miracles. This period was over when the apostles all died and the people they had laid their hands on died. At that point the miracles ceased.