(Full disclosure: I didn't read the full UN report, but I did read the UN Summary for Policymakers [1])

c0nducktr's claim "By trying to prevent them, we lessen their effects" is backed up by the summary. In particular, the summary addresses the following question:

What would happen if...

(A) we could magically go carbon neutral right now? (i.e. "what are the lingering/2nd-order effects of the all of our carbon emissions so far?")

(B) we curbed emissions so global warming[2] stabilized to 1.5°C by $TARGET_YEAR (without "overshooting" that goal [3])

(C) we "overshot" and let global warming exceed 1.5°C... but then got it down <= 1.5°C by $TARGET_YEAR

(D) we curbed emissions so global warming stabilized to 2.0°C by $TARGET_YEAR

It turns out that there are real differences between each outcome -- even between (B) & (C) (See A.3.2). Section B addresses specific differences wrt how bad things will get at different levels of global warming (mainly 1.5°C vs. 2°C).

For example:

(A) Immediate effects (B.3.1):
"Of 105,000 species studied, 6% of insects, 8% of plants and 4% of vertebrates are projected to lose over half of their climatically determined geographic range for global warming of 1°C, compared with 18% of insects, 16% of plants and 8% of vertebrates for global warming of 2°C (medium confidence)"

(B) Secondary effects ("Reason for Concern #5"):
"Large-scale singular events: are relatively large, abrupt and sometimes irreversible changes in systems that are caused by global warming. Examples include disintegration of the Greenland and Antarctic ice sheets."
If a marginal increase in the level of global warming causes a singular event to happen, then that increase causes a disproportionate amount of suffering via its secondary effects.

(C) Lingering effects (A.2):
"Warming from anthropogenic emissions from the pre-industrial period to the present will persist for centuries to millennia and will continue to cause further long-term changes in the climate system, such as sea level rise, with associated impacts (high confidence)"

That said, you are totally right that we still need to prepare for the effects, and we need to do it sooner rather than later. E.g. "(B.6.2) Adaptation is expected to be more challenging for ecosystems, food and health systems at 2°C of global warming than for 1.5°C (medium confidence)."

Unfortunately, global warming is expected to have disproportionate impact on some vulnerable regions/ecosystems (e.g. coasts & tropical regions). Some of these vulnerable regions are Least Developed Countries and/or small countries with correspondingly small infrastructure budgets (e.g. some island nations). On an individual level, it may be "advantageous" for less-vulnerable nations to focus on improving their own infrastructure, rather than reducing their carbon footprint. In economics, this "advantage" is called a "negative externality"[4] (aka "tragedy of the commons")... it's part of what got us into this mess in the first place.

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FOOTNOTES:

[1] https://www.ipcc.ch/sr15/chapter/summary-for-policy-makers/

[2] Note: the term "global warming" is an average value -- robust to normal fluctuations in temperature.
It's defined as the average temperature at the earth's surface over a period of 30 years. So the "1950's level of global warming" is the average surface temp over the period [1950 - 15, 1950 + 15] == [1935, 1965].

[3] "overshooting" here means "letting the peak levels of global warming exceed 1.5°C".
Note: if we let global warming[2] exceed 1.5°C for just a few years (and still get down to <= 1.5°C by $TARGET_YEAR), it still counts as "overshooting".

[4] https://www.economicsonline.co.uk/Market_failures/Externalit...