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.