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Their value depends on what you plan to do with them.  There is some evidence that sewn cloth masks work better for certain purposes and in particular situations.

Their value also depends on..….How well they are made
-A tighter weave of cotton……....Jersey tee shirt vs. high thread count cotton sheets
-Number of layers of fabric……..More is better, without restricting your breath too much
-A better fit.            ……………………………One size does not fit all

Their value also depends on how long they are worn and how well they are maintained.
Worse when wet, which can quickly happen with coughing and sneezing
Worse when you put them on and off wrong, contaminating with your hands

The classic surgical mask was originally made from cotton.  Surgical masks were designed to protect the patient from getting infected by the surgeon. They were not designed to protect the surgeon who wore it! 

Modern surgical masks block about 50% of the small particulate matter in air.
N95 Respirator masks block 95% of such matter when fitted correctly.
   Hence the name and why they are a gold standard, to protect the wearer.
A loose woven cotton mask blocks less than 5% of these same small particles.

The value of sewn cloth masks has been studied scientifically, but not very rigorously.  There are, however, some take home points from these studies.

Sewn cloth masks do not protect the wearer from viral illnesses very well, especially…
-With loose knits fabrics,
-When wet from breath and saliva
-When worn for extended periods
-When handled and washed poorly

Sewn cotton masks function best when worn by a sick person to protect others. 
But the same limitations apply – they must be made well, fit well and used well. Ideally, any such sick patient should isolate at home and not be out in the community. 

For a patient to wear a sewn mask to help protect a health care worker, when surgical masks are very scarce; they are likely to be better than nothing when worn correctly.

Whether a sewn mask is effective when worn by a sick person at home to protect other household members has not been studied.  It might help, but is no substitute for excellent personal and home hygiene and maintaining as much physical separation as the home will allow.

On the other hand, when used by a healthy community member to protect themselves from sick people in the community they are probably not very effective.  Perhaps they are better than nothing.  It may give the wearer peace of mind and that may have value. 

Wearing a mask may also remind others to maintain social distancing.  My biggest fear is that it may give the wearer a false sense of security, leading to risky behaviors.

The newest indication for wearing a sewn mask in public, which has lead the CDC to recently recommend significantly more wide usage, is based on two facts…

Firstly, we know that a significant percentage of COVID-19 infected persons have few if any symptoms; known as asymptomatic.  They are not aware that they are infected.

Secondly, we know that these asymptomatic COVID-19 infected individuals can be contagious and can therefore spread the disease to others.

The hope is then, that if everyone wears a sewn mask in public, this will include those few unknown people who are asymptomatic carriers of the disease.  And in turn, wearing sewn cloth masks will then potentially decrease transmission of the disease from them to others.

I sincerely hope this theory is correct, but it is certainly unproven at this time.  Likewise, I hope it does not lead to an increase in risky behaviors that are in fact, known to increase transmission of this disease.