Executive actions can be changed by Biden without consulting Congress. So, for example the restrictions on travel from mainly Muslim countries was done by executive order. This order can be reversed on Jan 20th.
Legislation passed in the last four years can't be repealed except by Congress, and repeal bills can be filibustered in the Senate.
So, for example the Defense Appropriation bill contains a clause
Imposition of Sanctions.--Not later than 30 days after the date
of the enactment of this Act, the President shall impose five or more
of the sanctions described in section 235 of the Countering America's
Adversaries Through Sanctions Act (22 U.S.C. 9529) with respect to each
person that knowingly engaged in the acquisition of the S-400 air
defense system referred to in subsection (b).
The President (whoever that may be) can't remove that requirement for sanctions against Turkey and there is a minimum time of 1 year before the sanctions can be removed. It can be changed by repeal (however unlikely that may be).
While the President can change an executive order, he/she can't always reverse the effects of that order. If the President had ordered a forest to be cut down, reversing the order won't make the trees grow back! Reversing the sanctions on Iran won't immediately restore trust in the USA by the Iranian government.
In the case of Iran, there are more than one sanctions regime in operation. Removing Trumps sanctions won't remove all sanctions from Iran, many of which date back to 1979.
The point is to send a message. It also forces action from Biden, which can be a route to attack him in the future. It is easier to find fault in actions than in inactions.