A sign in /out form is useful for thanking volunteers, requesting funding, showing funders how their $ was spent and follow up contact information when doing another ramp.
These are useful before the build to develop materials needed, during the build so everyone knows what is going on & after the build to show funders what was done.
Someone has to pay the bill. These document where the $ went or who has to be paid. They also help determine cost per foot of ramp which is a good planning tool for future ramps.