The famous Shri Bilvashtakam (v. 6–7) mentions :
Lakshmyaascha stana utpannam Mahaadeva sadaa priyam,
Bilva vriksham prayachchhaami eka bilvam Shivaarpanam.
Darshanam bilva vrikshasya sparshanam paapanaashanam,
Aghorapaapasamhaaram eka bilvam shivarpanam.
The translation :
Born from the heart of Goddess Lakshmi, the Bilva tree is ever dear to Mahadeva. So I ask this tree to offer one Bilva leaf to Lord Shiva. Even if (one) has darshan ( view) of the Bilva tree, and touches it, surely frees one from sin. The most terrible karma is destroyed when a Bilva leaf is offered to Lord Shiva.
It is also believed that Lakshmi, the Goddess of Wealth, also lives in the bel tree. Those who perform the puja of Shiva and Parvati devoutly, using the leaves, will be endowed with spiritual powers.
In Sri Maha Lakshmi Ashtothara Sathanama Stothram we find Goddess Lakshmi described as Bilva Nilaya the one who lives under the Bilva Tree.
“Bhaskarim bilvanilayam vararoham yesesvinim
vasuntharam mutharangham harinim hemamalinim “
Bilva itself means which kills all the diseases. As per Ayurveda all the diseases are related with the disturbed digestive system somewhere and Bilva corrects the digestive system in all aspects so it makes senses to call this plant as Bilva. Bilva is also useful in management of hemorrhoids and fluid retention in the body. It also increases the strength of the body.