There are many demands on our time. It sometimes feels like there's too much to do and not enough time to do it. And on top of all that, God wants us to do those things that will bring us closer to Him: reading our scriptures, attending temple and other worship services, serving others, etc. It's not hard to understand why we sometimes let those things slide. But when we prioritize them, they help all the other activities in our lives to fall into their proper place.
When Jesus told Martha that "one thing is needful," He was not implying that the other things she was trying to do were unimportant or not worthwhile. Rather, He was pointing out that at that time, Martha's desire to learn from Him in order to draw nearer to God was more important. Concerning herself with keeping house and serving her guests was a worthy desire. But Christ would only be with them for a short time, and they were better served by taking advantage of His presence.
This doesn't mean every second of every day needs to be spent on studying the Gospel. We need to take care of our mortal bodies (which includes appropriate intervals of rest and recreation), maintain close relationships with those we love, and contribute to our communities and society at large. "To every thing there is a season, and a time to every purpose under the heaven" (Ecclesiastes 3:1). If we ever feel overwhelmed with all the things we need to do, or are ever unsure of what to do next, we can take some time to ponder what one thing is most needful in that moment.