What is Salvage Value in financial term ? why opportunities cost need to be calculated for any fixed assets?

For example, with a straight-line basis, an asset that cost $5,000 and has a salvage value of $1,000 and a useful life of five years would be depreciated at $800 ($5,000-$1,000/5 years) each year. The estimated value that an asset will realize upon its sale at the end of its useful life. The value is used in accounting to determine depreciation amounts and in the tax system to determine deductions. The value can be a best guess of the end value or can be determined by a regulatory body such as the IRS.