Trisher just because you are doing the same job as an employee of the organisation does not mean you are taking a paid employee's job. The funding of the organisation probably means that they can only afford to employ so many staff to undertake the work they do and after they have employed those staff, any further work, if it is done at all, needs to be done by volunteers.
I worked for a charity as a volunteer alongside employees doing the same job. I was working as a Case worker, visiting clients at home. The Charity had a grant from the LA that paid for 4 full time case workers, who had to give preference to client referrals from the LA, but many more clients asked for help than these employed caseworkers could possibly visit. As a result the charity recruited and trained a team of volunteer case workers who took up the slack.
If I and my fellow volunteers had not taken on the workload we did, these other clients would simply have been left without assistance. The volunteers only took on as many clients as they felt we could manage and this varied from person to person.