One reason to sow a tiny mustard seed is the teaching of Matthew 13.31-33, 44-52. The mustard seed is “like the Kingdom of Heaven”. It starts very small - with one act of faith, one prayer to God, one word of the Bible - then it starts to ‘mushroom’ to a ten foot high plant.
Jesus elsewhere teaches that "faith as small as a mustard seed" can do huge things out of all proportion to its size. Faith supercharge outcomes. Jesus's image of insignificance is the mustard seed. This seed of the Kingdom grows inexorably in the good soil of a true heart, until it takes over someone’s whole life, and becomes its entire focus. It becomes a “tree” in which others can find shelter. This even happens to a nation that becomes a mustard tree : people will flee to its protection. There is a relevant prophecy of the Old Testament and that the House of David would offer shelter. “Wild birds”, those fleeing predators and the storms of life, will come to rest and roost in this tree, says Jesus. Why the mustard seed/tree? Isn’t a mighty oak tree better? Acorn to mighty oak? No because the emphasis is on the tiniest seed. This is God's encouragement for the weak and insignificant, for those with no resources or strength, for those with nothing to offer. God can make even the weakest, a pillar of the nation : He made a village carpenter who who never attended Harvard, Oxford, Cambridge or the London School of Economics, the one figure who still dominates human culture and history. Jesus Himself is "the mustard seed", par excellence.
This fits in with all New Testament teaching. God looks with a special purpose on the weak, to build them up from nothing, due to their faith. He will magnify our weakest and smallest effort if we will magnify Him. God gives the growth and strength to the mustard seed which is tenacious - it just keeps going and pests do not like it. It is also useful in its seed for mustard and you can eat its leaves.
Prayer request : the church in Iraq that is being massacred. A mustard seed prayer can be powerful on its behalf.