TL/DR: Invocation of article 155 is an extraordinary measure and almost everyone wants to keep it short. Elections offer a possible solution to it. There are some reasons to hope for a different electoral result but if electoral results are a repeat of 2015 the Spanish government can keep article 155 in place.
The main reason would be that it offers a (possible) exit from the invocation of the 155. Article 155 is intended as an extraordinary measure, to be used just as needed. And most analyses about it have been very explicit about the lack of experience meaning that there is no clear way to decide the proper way to do it. A newly elected Parlament offers a clear, definite milestone with an important degree of legitimacy and that most Spanish political parties may agree to.
It looks quite likely that the current results from the Catalan regional election, 2015 would be still quite equal
Well, this brings us to At those elections, JxSi & CUP together won 47.8% of the votes; that got them 72 seats out of 135 (majority at 68) but they were not the most voted option.
Those electoral results and recent events can give hope for1:
Even keeping those results, unitary lists by non-independentist parties would give them the majority.
Due to the actions of the current Catalan government polarization has increased. People will be more mobilized to vote2, and those who were opposed to the independence movement but wished not to vote may decide to vote now that there has been an actual attempt.
JxSi & CUP campaigned on "we declare independence, everyone in the world recognizes us, the EU says that we will stay in, the central government will just have to recognize us, profit!". Well, nothing of that has happened3. More in detail:
Absolutely no international recognition, backing of the Spanish government by the EU, and a clear message that the EU will not incorporate a unilaterally independent Catalonia.
Part of the idea was that business and economy would love and profit from independence, but it turned out that many business have elected to signal that they are not comfortable with the idea of an unilateral declaration of independence. While the actual economical impact of those moves may be low, it can worry many potential supporters. There are also reports of a slow down (up to 20%) in tourism bookings and some investments being delayed. That will not suit well some of the people who supported JxSi.
As the independence movement promised success, there was no issue breaking Spanish laws. But right now Spanish law is still in force, and there are judiciary investigations in place about possible felonies and crimes that could have happened. Some people may be charged. New MPs might take that into account and be more careful in the future. Lower level positions may be aware that they are even more at risk (no Parliamentary protection) if they obey illegal orders.
As it is said "Power wears people down, but not having power wears them down more"4. Faced with a lack of positive results from the expected plan, the drawing of new plans and objectives could lead to tensions between the more die-hard and the more moderate sector of the independentist movement surfacing. And in a new election, the only hope for each independentist party to grow is to steal votes from the other party.
And of course, the important point is that nobody in the Central government has said that new elections will automatically untrigger the 155. It could be very well that, if the new Parlament is a repeat of the current one, that the 155 is kept in order. It would certainly mean that one of the possible solutions to the crisis has been lost, and a great disappointment to the Spanish government, but it would not be a fatal blow.
1Let me state that again: those are things that can be hoped for. I am not claiming that they will happen, but I think they are reasonable.
2Although the participation in the 2015 elections was rather high (74.9%), it is still low when you compare with independence referendums in Scotland or Quebec, so if the election becomes a single issue one it could be expected to be higher.
3Well, some days they say independence has been declared, then the next day they say it has not been. So... maybe 50% of the first one?
4"El poder desgasta, pero el no tener poder desgasta más"