Build rate is important in this game as it affects how fast something is built. Time is often seen as the most important resource in Supreme Commander, and the faster you can get a unit off the production line, the faster it can get to doing its job and the faster the factory can start working on a new one. The larger the number the faster the building/unit is made. A build rate of 30 is better than a build rate of 5. However the higher the build rate the faster resources drain. This may not seem a problem when building a wall section, but if you were building an Experimental unit then this would have a bigger impact on your economy.
You may think after seeing this table "im not going to build as many T3 engineers and use moreT1 ones instead". This is a good idea when assisting factories or experemental building but there is a few problems with this.
1) They bump into each other. Sometimes you are better off using less enginers as they just make a mess of pathfinding and slow stuff down. this is true when building small cheap structures like T1 pgens or T2 massfabs or wall sections. so this is why you should use hives and kennels, T2 and T3 engineers and SCUs or acu with buildrate upgrade.
2) Veterancy. When an enemy army walks in on you or an experimental those massed T1 engineers will feed it veterancy. When an experemental vets up it gets 10% more hp and a 25% heal. T1 engineers will keep it fed long enough to destroy your whole base. Another reason to use hives, kennels, T2 and T3 engineers and SCUs or acu with buildrate upgrade. one way to avoid this is Ctrl-K them when an experemental gets in your base. (See photo below)
3) When assiting land factories the engineer can get in the way of the units as they get off the factories, slowing down the buildrate. So you have to make sure they are not in the way manually.
You will learn a few things form this table. For example it is cheaper to build new TML silo than to assist an allready existing one. You will also see how bad the mass/buildrate ratio of SACU's actually is. Do not forget that while T1 and T2 factories have good mass/buildrate ratio compared to T3, T3 factories can make T3 units. So next time you are making a land army, always make factories with the same tech level of the units they are producing, to be more efficent.
On the other hand, you want to consider aspects of a building unit other than the mass/buildrate ratio:e.g., the atlantis with its 50 mass/buildrate is very good because you also get a very good submarine and it can hide your air force. SCUs may seem REALLY bad, but keep in mind that they can reclaim and build under fire while other engineers cannot. So they may pay back there mass cost quite quickly.
Here is a little table to show which units build at which speeds and mass cost per unit of buildrate: