When you re-format, it will erase ALL data on there. You may possibly add maximum extra storage space by reformatting, BUT it comes with several catches. Most of the time your OS will partition your hard drive for different uses. Most common is a recovery partition, which I would strongly recommend leaving that alone unless you have an alternate back up option in place. You can look at your hard drive partitions in Win 10 by going into disk management. If your recovery partition is for example 15Gb, then you could gain an extra 15GB of usable hard drive space by removing that partition, at the cost of having no redundancy or backup. If your hard drive is 300GB, and 15gb is reserved for recovery, then you only have ~280Gb usable. There is no possible way to get more than 300GB of storage out of a 300GB hard drive though. What you can do is get an external hard drive, and use that for storage
Also, you can repartition your drive without doing an entire reformat. Just open disk management, right click on the partition and click shrink or expand volume, depending on which partition you have selected. Make sure you know exactly what a specific partition does and if you can live without it, before you get rid of it.
If you are short on space, I would say to use some software like WinDirStat to see exactly where your memory is being used up, or just go buy an external USB hard drive for more storage