Insulation on resistance wire, what's the use?
Resistance wire is used to make heaters (usually alloys such as Nichrome or Kanthal). This type of wire is refractory (has a high melting point) and is usually resistant to oxidation so you can get it very hot in air without undue shortening of the life. Sometimes it is shoved into a fiberglass braid or ceramic braid but usually the wire maker supplies it bare. 

Resistance wire is also use to make wirewound resistors (typically different alloys such as Manganin or Constantan). It is typically insulated. Such wires are not intended to get very hot- the advantages of the alloys used are that they have a low TCR (Temperature Coefficient of Resistance) near room temperature up to perhaps 100°C). The insulation may be an enamel of some kind, polyester or some similar thermoset or thermoplastic, depending on the intended application. 

The two classes of alloys are quite different and the latter probably won't make a very good heater and the former won't make a very good resistor.