Although I can't give you a definitive answer with regards to the use of embroidered purses, something to consider is the fact that many women's purses are not seen in art anyway - due to the fact of them being worn being the kirtle and gown. This could be the link - that many very decorative purses are womens. But, if you look really closely, you can sometimes see parts of women's purses which seem to show decoration or highlights - which could be either embroidery or brocade silk ground. (15c paintings of the Virgin with Elizabeth often show the Virgin with her gown hitched up a bit, and you can often spy a pouch hanging there in the shadows).
There are actually a very large amount of these types of purses - embroidered, using fancy brocade silk grounds, etc which survive (more than you would first think) and this is because many were used as relic purses, so survived in Cathedral collections. I assume that gifts were given to the Church in purses such as these. That doesn't mean that the purses were made soley for this use of course - in the same way as fancy fabrics were given, they may have been purses that were in use prior to being given.
I would doubt that some of the examples would have been used to give alms to the poor - the embroidery itself (and inclusion of decoration such as pearls and gold threads) would make the purse more expensive than the alms. Also, there are quite a few examples with integral, tiny little coin purses.
So, they may be women's purses. (most likely in my opinion). They may also (some of the much smaller ones) be coin purses to use within a larger purse. They may even have been created originally as a 'wrapping' for an expensive piece of jewellery - something still done today.
Opus anglicanum is a style of embroidery, (not a purse)using gold threads and fine stem stitch. This beautiful and highly skilled embroidery is found on many items, surviving examples are mostly ecclesiastical. The English were highly skilled at this method of couching the goldwork (which gave it quite a bit of flexibility by all accounts) and on good examples the faces have an almost 3d quality. Hence the 'anglicanum'.
You may find this site helpful -
Medieval Silkwork
Both Isis and Machteld are doing alot of work with medieval embroidery, and have quite a few examples of their work (with links where possible to images of originals). One of them may be able to shed light on your question about using a lesser quality thread to embroider with.
Hope this helps a bit.
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Gina-b Silkwork & Passementerie
Tak v Bowes Departed
Soper Lane