|© 2006 Jeff Belmonte, Flickr | CC-BY | via Wylio|
As far as I am aware, this is the only Bible verse that has a direct command to newlyweds (feel free to correct me in the comments if I am wrong!). Though this verse was intended specifically to be directed to the Israelites and how they should run their society, the principles of this verse are instructive to modern-day believers as well.
Commentators say that there are likely two main reasons for this command. First, so that newly wed couples could have time to develop their relationship without distraction. Secondly, so that they would have time to conceive and bear a child early on. Many newlyweds would do much better in their relationship if they would heed this advice.
Take time to develop your relationship. When you first get married, it is important to continue pursing building your relationship. Indeed this should be happening during the entirety of your marriage, but to do so in the first year is laying a good foundation for your future years together. Make it a priority to go on regular dates, weekly or every other week at least. Look for things to do together; it is not enough to simply live together under the same roof, but it if important to really live life together.
It is also important to avoid major changes and other things that can put strain on this foundational year of marriage building. Many couples marry and then move right away to start a new job or grad school. It is very difficult to lay a healthy foundation for marriage while also trying to find a new church, make new friends, and figure out a new city. It is worth putting off a job or grad school for a year to stay put in your familiar area to make sure you can focus on your marriage.
Make babies. Early (and often!) God tells us to be fruitful and multiply and that children are a blessing. Couples who reject the modern philosophy of children being a burden and choose to obey this will receive God's blessing for obedience. Having children makes a marriage stronger as it gives husband and wife something to work on as partners: a ministry that involves their whole lives together. Having children is also incredibly sanctifying, which has the added benefit of making a marriage stronger as man and wife are humbled before God and seek him more and more for godly character and help to be good parents.
It is important to remember that the advice to wait to have children is coming from a society that kills 1.3 million babies per year, doesn't honor God and has an incredibly high divorce rate. Believers choose to follow God and do things differently, because we know that the ways of God are higher and better than the ways of man. This includes seeing children as a blessing ALWAYS (not just when they are convenient), which practically means having children early and often while the world says wait and stop.
When thinking about this verse, I realized that ALL of the Christian couples I know who are already divorced chose not to have children in the early years of their marriage. On the other hand, most of the couples I know who have mature walks with God and good marriages had children very early on in their marriage. I know that this is not a scientific study, and that correlation is not causation, but it is interesting to note that I have seen in practice how this does affect a relationship.