It is said, that those who speak the truth, come what may, the truth starts speaking itself in them. Jesus said, if I am in your words and your words are in me, then they will come true.
Now, coming to the explanation. If you always speak the truth, and nothing but what the truth is as far as you are concerned, then a time will come when whatever you speak casts itself in the realm of reality and indeed becomes the truth. And no, this isn’t the only reason why you should only speak the truth.
The above mentioned explanation is the cause of the action, or a reaction as they say. The reason is simple, God wants you to speak it, and thus you must. And the reward of that obedience is a tongue of gold. Nope, it won’t happen overnight, and is unlikely to happen even over a lifetime. But as time passes and as events unfold, the sun will rise and your words will one day come true. But.
It may take several lifetimes to reach that stage. And by that time, chances are most such people would know what to speak and what not to. But all that you say will be answered then, whether sooner or later. And you would have moved a notch up in the realm of spirituality. And of course, you would have received blessings of people in the name of being honest, truthful and pure.
This is the kind of tapasya Lord Shiva did (not the same tapa though) for thousands of years before he finally ascended to Godhood. He was born a man like you and me, but he never died and death or life is forever redundant to him now.
There are probably numerous other cases in history and especially in mythology when practicing something diligently leads to being benefited with certain rewards. Although in the present age, these rewards might not appear at the surface as often, and often might be subtle to see, nevertheless, it is far easier to attain nirvana today as compared to what it used to be in the Golden ages. Trust me, this is what saints today yell. Earning spiritual points today is easier, despite the fact that most of the world is studded with Cyclops of the disguised Satan.
And truth is something which will get you honor and respect more often than not. And if it is done from the heart, then all else becomes secondary.