Why Should We Speak Truth?

You've asked a question about speaking truth and you want to know that why should we speak truth. You must understand one thing that speaking truth is one of the best qualities anyone can obtain. There are also several benefits of speaking truth. Some of them are that if a person speaks truth he does not need to hide the correct and real information any more and contrary to this if a person tells a lie then in this case he will have to tell more lies to hide his old lies. Hence this situation would be confusing for him and the other people will also come to know about the fact one day.

And that thing would cause him to lose respect in the eyes of people. So if you're trying to speak truth or you speak truth it is a divine blessing upon you by your God. And you must not forget that the truth is the only quality which is commonly considered as a good quality all over the world. And there is a famous proverb of English language about the truth that truth will out. This proverb is telling as that we do whatever we want but one day truth will be known and we can't keep it secret.
I hope this answer is sufficient.

Being truthful always helps. If you are lying, in order to cover that lie you will lie time & again.

If you are truthful there will be no problems

Telling the truth goes to the very heart of one's character.  If you are found to be a liar, you will never have the trust of those with whom you interact.  You lose credibility and respect when you are found to be a prevaricator (look it up).  One lie frequently leads to another and another ad nauseum.  It's hard to keep up with them.  You'll eventually get caught.  An honorable person does not lie.