This is basic construction sites for your country. More workshops will allow you to build more buildings per turn.

Workshops & building rate

Building rate is based on the number of workshops you have.

They also provide 30 residential space for your population (vs 10 for wilderness).

The more workshops you have, the faster you can build any building
- EXCEPT barriers (which are magical things that always 1 turn to build, not influenced by workshops.

formula

NOTE: To get nice round numbers of buildings per turn, build workshops up to x9: that is, 299, 249, 319, etcetera.

Early in game:

At the beginning of a game, it is worth it to build workshops one turn by one turn. That way you go MUCH faster from 50 to 400 workshops than by building 350 works in one big bunch of turns.
Not just build workshops, but also watch your population and geld while building them: you want the population to grow fast, so from time to time you also have to build Farms and Towns in small amounts - Optimally, you keep your population at 90% of Max. Population at all times. You need all the geld you can generate to keep building and building early in the game. You don't want to go bankrupt from only building workshops and forgetting that you need population to generate geld to upkeep workshops!

Later in game:

A high number of workshops are essential if you loose and reconquer a lot of land often (aggressive play, war).
- Thus: keep them at 300-400-ish ammounts.
- Stable players can do with a 100-200 less workshops.
If you have a lot of workshops (think >400) and sufficient geld: you can ride out spells like Death and Decay and/or Meteor Storm through turn-by-turn building about the amount of buildings that these spells destroy each turn. If you have multiple building destroying spells on you, it is better to first dispell one of them.
Loosing your workshops through spells or bankruptcy should be avoided at all costs - it takes dozens of turns to build them back to decent ammounts - which means you lose a day worth of turns.