In all the years I've spent in the gym, nothing has had a greater effect on my fitness than tracking and monitoring my progress through measurable, data-driven training.
By collecting quantitative (numeric) data by performing measurable, observable and repeatable activities (for example, both 5 reps of a back squat for max load or rowing 1,000 metres as fast as possible on a rowing machine will provide quantitative data points), any gym goers can track their progress using cold hard facts rather than staring in the mirror and wondering if they are looking 'more toned' than last week.
By having quantitative data points to base our training on, we can guarantee progress through scientifically based strength and conditioning protocols, such as progressive overload, where we gradually increase the stress placed on the body in training through sub maximal (percentage) work, which can be determined only by having data to begin with.
Wandering around a gym aimlessly using random machines or just jogging endlessly without a specific goal or plan will just result in slow or no results, and when or if you do get results, inevitably you're bound to plateau. This seems like such a basic principle: track your workouts, gradually make them harder and you'll get results, yet you'd be surprised how many people I come across with no idea what their maximum number of pull ups or fastest 2 km run time is. (These are just two examples of data points, but you can pick any movement, rep scheme, load or distance to measure.)
Without tracking these data points, how can we tell if we are moving towards our goal or away from it? Invest in a training diary or simply jot your sessions down in the notes section of your phone. Log your weights lifted, reps completed, and the time it takes for you to complete whatever conditioning you do, and then aim to improve these things over time.