In the summer time, the levels of humidity in your home probably isn't an issue for your indoor plants. My apartment naturally stays about 50% in the summer, which is perfect for my plants. As the winter comes along the humidity levels often go down to as low as 20 - 30%. 

I noticed around fall to early winter time last year,  my fiddle leaf figs, monstera, benjamin ficus were not doing so well. They were dropping leaves, turning brown or yellow. I thought that it must be because I hadn't been watering them with close attention. 

A little while later, I bought a humidifier for other another reason (DIY hot yoga room). I noticed that my plants seemed to be doing better when I used it more often, so I  researched the preferred humidity levels for indoor plants which is typically around 40-50%. 

There are of course other ways to increase humidity levels, other than buying a humidifier. 

• Spray plants with water. 

• Put trays underneath plants with pebbles and water. 

• Set bowls or glasses of water out (I always leave a cup of water around my ferns). 

• Let your laundry hang dry in your home. 

• Group your plants together, the more plants in your home, the easier it is to sustain humidity levels.