Or is it just to save power?
Yes, it is to save power. The Intel Processor
U suffix stands for "Ultra-low power"
See my answer Meaning of Intel Processor suffixes for more information on how to compare processors for performance and power consumption.
Note also this comment by misha256 on the above answer:
A high spec mobile CPU can outperform a low spec desktop CPU no problem so the suffixes don't help you there. Likewise I've seen mobile CPUs that can burn through more watts than a low/mid range desktop CPU.
What do the suffixes mean?
C - Desktop Processor based on the LGA 1150 package with high performance graphics
H - High performance graphics
K - Unlocked
M - Mobile
Q - Quad-core
R - Desktop processor based on BGA1364 (mobile) package with high performance graphics
S - Performance-optimized lifestyle
T - Power-optimized lifestyle
U - Ultra-low power
X - Extreme edition
Y - Extremely low power
About Intel® Processor Numbers
The processor number is one of several factors, along with processor
brand, specific system configurations, and system-level benchmarks, to
be considered when choosing the right processor for your computing
A higher number within a processor class or family generally indicates
more features, but it may be more of one and less of another. Once you
decide on a specific processor brand and type, compare processor
numbers to verify the processor includes the features you are looking
See the following link for all the
Product Line suffixes.
Source Intel® Processor Numbers: Laptop, Desktop, and Mobile Device