Firstly you have to think about how the liturgy was undertaken in religious buildings of that time, at a basic level we can think of monasteries and significant churches in the same way at that time. Worship at the high altar was a closed affair, screened off from the eyes of the "public" with veneration of the mass undertaken only by monks and priests. To be very simplistic: you can think of the word "service" as having a literal meaning in that form of worship - you or I would pay the priests to undertake holy worship on our behalf. In addition to our own personal prayer or our own devotions to smaller altars in the main church or monastery we're paying the high priests to perform the heavy-duty devotions on our behalf.

The richer we are the more we pay the priests. If we want prayers said (more often sung) for ourselves or a loved one after death then we make a substantial payment or bequest to the chantry of the church or monastery to have that done regularly, possibly in perpetuity. That affords us several things: we cleanse ourselves of our sins (the more we pay the better the wash) and we gain protection in our daily endeavours, and we ensure a heaven-bound afterlife. If we're a wealthy individual then the chances are that we're doing a bit of fighting for national or domestic interests so we need a good holy insurance policy - we might even be so important that a senior cleric would ride into battle with us to protect our troops. A good win would mean a good donation to the cleric's altars.

Those payments for services and chantrys form the income for churches and monasteries although they also undertook other services. Very often they were "hospitals", places that tended for the sick or the dying, activities which could generate revenue on balance with wealthy "patients".

However, not all payment came in monetary form. It isn't unusual to find grants of land given to monasteries. Naturally they had mouths to feed and backs to cover so a monastery would require space to keep livestock for food, milk and wool, and fields where they could sow grain crops. The primary food source was fish rather than meat so monasteries would also have a number of large fish ponds to farm from. The output of monasteries could easily exceed consumption and so the excess would be sold. Many monasteries became rich on sales of wool, saltfisk, ales, meat and the like.

The head of the monastery would control the finances and so by the 10th and 11th century many bishops and archbishops were astonishingly wealthy and powerful political figures in their own right. There was competition between monasteries and orders to have the ear of the monarch and other important figures with a view to gaining more land and more opportunity for profit. Some monasteries would rent land to tenant farmers for a share of output or for a cash fee.

In summary: monasteries would receive gifts of cash and land that proportionally increased the holy credit of the giver along with the capital value of the monastery.

Sources:

Medieval Monasteries, Greene J, 1992.

Monasteries in the Landscape, Aston M, 2009.

Understanding Medieval Liturgy, Gittos H, Hamilton S, 2011.